Crack to Basics: How to Fix Up Broken Aquariums

Posted by Augusta Hosmer on

Read Time: 13 minutes

Welcome to part two of all you ever wanted to know about working with used fish tanks! In part one, we told you how to find them, buy them, and clean them. In this part, we'll show you what to do if you either buy a used tank with damage or an existing setup gets damaged. 

How to Prevent Damage in the First Place

The easiest way to keep your aquarium from getting damaged is to put it on the right stand, especially if you have a top-heavy vertical tank. Keeping the tank level, all the bottom edges supported on a surface that won't scrape/chip them, and the water pressure evenly distributed prevents damage caused by stress on the tank structure. People underestimate how heavy a filled fish tank can be, even a small one, so make sure you have a stand that can handle the weight. To put it in perspective, a gallon of water weighs just over 8 pounds at room temp. Add the weight of the tank, and even just a little 10 gallon filled up can be almost 100 pounds (and that's without gravel and accessories). On that note, never move a fish tank filled with water, because you can damage it that way, too. 

A planted fish tank on a too-large metal stand

See this? Don't do this. This is one of my earlier tanks. Tank is nice. Stand is nice. They are not nice together - see how the sides aren't supported? This puts extra stress on the tank. It survived like this for a while, but only because it was a smaller tank.

Choosing a good tank location also prevents problems. If you have pets or kids, securing the tank and keeping it out of reach when you're not there to supervise can keep it from gettting knocked over by a zooming cat or a stray flying toy. Keep them away from doors, and consider keeping them in a location where they're visible and accessible, but isn't in the way of your typical traffic patterns. Don't beat yourself up if this doesn't work, though, because kids and pets are gifted at getting into things. 

You can also choose different tank materials for the damage you're most worried about. You can prevent scratches by buying glass tanks. Glass is popular because it's cheaper and it doesn't scratch easily, but it is heavy and fragile. To prevent cracks and breakage, you can get acryllic tanks - these are often advertised as shatter-proof and are much lighter than glass. However, they're often more expensive and it feels like they can get scratched up just from thinking about scratches. 

How to Repair a Cracked Fish Tank (Empty)

We'll start with the smallest headache. Most people think if you have a crack in your aquarium glass you have to start over completely with a brand new tank, but you may not have to. Would it be better and less risky? Yes. But if you're as broke as your aquarium is right now.

You'll need some combination of the following, depending on what you decide to do to make your repairs:

-Aquarium silicone - Make sure it's actually aquarium silicone and not just regular silicone, because you want it to be safe for your fish, waterproof, flexible, and more resistant to algae/bacteria. You can find it for less than $20, and it typically comes in clear, black, or white in a tube. Double check the curing/drying time for the brand you're buying and make sure it'll be okay to use in the time frame you need (some really specialized silicones take weeks to cure. Pretty sure you don't want to deal with that). 
-Scrap glass/extra pane of glass or acryllic - Even just a piece of glass not from an aquarium works if it's big enough to cover the crack plus at least a few inches for a patch job. It doesn't have to be a perfect square or pretty, normal shape unless you want it to be. If you're replacing an entire pane, though, you want it to be identical to the existing panes. 
-Something to clean the glass with - In part one, we mentioned diluted white vinegar or diluted bleach. Those will both work as long as you clean them off sufficiently with water as your final wipe off
-A razor blade/box cutter - Or anything sharp that can be used to cut excess/old dried silicone
-Tape measure
-Rags/Paper Towels
-Something to apply/spread the silicone and/or gloves if you don't want it on you- 
personally, I'm one of the messiest human beings on the planet and I don't care since the silicone can't hurt me, but if you're not like me, maybe add this to your list.

Three tubes of aquarium silicone


Option One: Patching the Crack with Extra Glass, Without Replacing the Entire Pane

For a smaller crack only affecting one side of a glass tank, you can actually patch it without too much work, expertise, or expense. You just need your aquarium silicone, some scrap glass/acryllic, something to clean the glass with, and something sharp to cut with. The upside: it's cheap and easy even if you've never done a DIY project in your life, and you can usually get a damaged tank to hold water without having to replace the whole thing. The downside: just like a patch on a piece of clothing, it will be visible. If the crack is in a location where you can hide it, like towards the bottom or in the back, this might not be an issue. I repaired a 40 gallon with a very noticeable crack and a horrifically obvious patch job...but it was closer to the bottom and I was able to stick a few java ferns in front of it. No one knows it's there unless I point it out. You can also get a 3D tank background and hide it behind that. But if it's out in the open, it could be pretty ugly-looking, and you may end up making it a temporary thing while you find a shiny new setup.

Step 1: Drain the tank if you haven't already, and clean it out. Even if you don't deep clean the whole thing, you want the damaged area you're working with to be as clean as you can get it before you seal it. Use the vinegar or bleach like we talked about, and finish with water. Let the tank dry out completely. Make sure the scrap panes of glass/acryllic you're using are clean and dry, too. 

Step 2: Apply silicone to the crack. If the damage goes all the way through the pane, I like to do this on the inside and the outside of the tank and make sure the silicone really gets in deep (don't worry if you have silicone on the outside glass. It looks ugly now, but you can scrape off the excess later). On the inside, it especially doesn't matter how it looks, because you're about to add more silicone and some glass/acryllic anyway.

Also, yes, a lot of aquarium silicone smells like vinegar, and that's normal. It contains acetic acid. This is actually a good thing, because you can use that as a way to gauge how far along the silicone is in the drying process (still smell vinegar? You're not done, yet). The smell can get overwhelming after a while, though, so work in a ventilated area if you can. 

Step 3: Apply silicone all over the piece of glass/acryllic you're using to patch. You can really smear it on there. When it dries, it will maintain the shape of however you applied it, so make sure it's how you want it. Just know the more you apply, the longer it will take to dry. 

A flat sheet of acryllic with silicone being applied

Step 4: Press this piece against the crack on the INSIDE of the tank, so that the glass covers the crack completely and also has at least a few inches of extra space around it. You want the whole space to be sealed tight with silicone so that in the event part of it does come loose and water tries to leak out, it can't get to the crack. Some silicone will squeeze out as you press the two together. Wipe/scrape that off as needed. It's important to put the scrap glass on the inside of the tank so that the water pressure is working for you by pressing the glass even more firmly against the crack and strengthening that seal. 

Step 5: Wait for it to cure. Most of the time, silicone needs at least 24 hours to dry completely. The longer you can wait, the better. If you can repair the tank in a warm, humid environment (seems counterintuitive, but it makes silicone cure more quickly), that can help. Don't move the tank during this process.

Step 6: Test it, test again, and test again. Fill it up and see if it still leaks, and make repairs as needed. You want to be sure it's actually sealed before you even think about adding fish. 

Step 7: Scrape off excess dried silicone (without pulling the seal loose), and clean the tank and surrounding area up. Set up your aquarium like you would any other, but monitor it more closely than you would a new one for water loss. 

Option Two: Replacing the Cracked Pane Completely (Don't  Do This Unless You Have To)

This is not something I like to do or recommend because of how much effort is involved and how many things can go wrong. If it's a small crack, I'd just patch it instead. If it's significant enough I need to consider replacing the whole pane to prevent a blowout, I'm probably more interested in finding a new tank at that point. 

But, in a situation where you're really attached to a particular tank and/or they're hard to replace, you don't have access to a new tank and need one to hold water sooner, or it will be insanely more expensive to buy a brand new one because it's massive, you can try this. Just know you will need to test it out a lot and watch this tank like a hawk...and that you're going to mindlessly hate old silicone by the end of this. The upside to replacing the whole pane: if you can get it right, it can be cheaper than a new tank and no one will be able to tell it was damaged. The downside: It's a pain (or should I say...pane), especially if there's trim on it, and it's even more likely to leak than a patch job.

Step 1: Clean the tank and replacement pane, then see if the tank has a trim (black plastic around the edges in most modern tanks). Sigh in relief if it doesn't. Yell your favorite swear word and reconsider your life choices if it does. Pray to any deity who will listen that, if the tank has trim, it a) isn't holding the tank together and is just there for looks, b) isn't one solid piece, and c) was attached by a sickly Victorian child who didn't get paid enough to care if it stayed on. If it doesn't have trim, relish in the feeling of being God's favorite and skip to step 3. If it does, sacrifice a virgin to the aquarium spirits to gain their favor and go to the next step. 

Step 2: If there's a frame on the pane you're replacing, you now have the unbelievably fun task of ripping it off without further damaging the tank or breaking the frame if you want to put it back on. If it's holding the tank together, I wouldn't even try it. If you don't want to put the frame back on and are going for a rimless look, it's easier. Keep in mind the bigger the tank, the riskier it is to remove the additional support of the frame and bracing/resealing may be needed if you do.

You can cut the frame into smaller sections to make it easier to work with. Use a razor blade or box cutter to cut/remove the old silicone holding the trim on and carefully pull it off (Vinegar or heat can also be used to soften silicone). Thin blades are the best. If you're keeping the frame condolences. It's the same procedure, but it takes much longer. Removing trim is not a difficult process, but it takes time to do it right. Also, a lot of times the frame hides sloppy silicone underneath it from the manufacturer, so you may have to clean that off, too, if you go rimless.

Step 3: Remove the old silicone from between the panes, remove the damaged pane, and clean the edges. Carefully position the new pane and support it with braces, tape, or whatever works best for you - just make sure it doesn't move. Having someone else around to help you hold/maneuver is a godsend. You can try just replacing the silicone where the new pane attaches, but since new silicone doesn't attach to old silicone, you could end up having to reseal the other edges of the tank, too (more details on that below).

Step 4: Add aquarium silicone on the inside and outside of the pane edges and smooth it out to fill the crease. Don't worry if this gets messy - you can trim off the excess later since silicone dries in a rubbery texture. 

Step 5: Wait for the silicone to cure. The longer you can give it, the better. 

Step 6: Test it out with water and keep testing. Also, if your tank doesn't already have braces, think about adding them for extra support now that you've ripped it apart and put it back together. If you've removed a trim, European braces could work well.

How to Fix and Add Aquarium Supports/Braces

The most common damage on fish tank supports, especially on larger tanks, is a broken cross brace. You don't see them much on smaller tanks, but if you have a plastic, metal, or glass/acryllic beam or panel across the center of a larger tank, that's what it is. The bigger the tank, the more support it needs. This, fortunately, is a fairly easy fix. If you want more support on a tank that doesn't have them, center braces, European braces, and corner braces are also an easy addition. 

A glass aquarium with a center brace indicated

To repair a broken center brace without replacing it, carefully brace the front/back of the tank until the edges of the broken brace meet (A bar clamp will be your best friend). Use silicone or epoxy to reattach the pieces and wait for them to cure (I like epoxy better for braces). For extra support, you can also attach a patch sort of like how you did for cracked glass over the area with another piece of the material you're working with and more epoxy/silicone (just make sure if you have a tank lid it doesn't interfere with it).

To completely replace or add a center brace, use a piece of material like glass, acryllic, metal, or plastic at least the same thickness as your tank glass. Remove the old brace and silicone. Measure the length of the brace to fit the inside of your tank and make sure it's at least a few inches wide, and use silicone to attach it across the top of your tank underneath the trim, if you have trim. Keep it supported and wait for it to cure.

How to Fix Leaking Aquarium Seals

Great, there is no cracked glass and all the supports look good. But you can see water seeping from the edges of your tank, or a puddle has appeared without any obvious damage. Now what? If there are no cracks on the bottom of your tank beneath the substrate where you can't see them, it's probably a seal issue. So grab your silicone, and let's get into it. It's similar to the first part of replacing a pane, but without the agony.

Step 1: Same as above. Clean the tank out, let it dry. 

Step 2:  Make sure the tank walls are supported, then use a razor to scrape off the old silicone inside the tank. Old silicone will not bond to new silicone as effectively, but you can try it if you'd like. The bond just won't be as strong.

Step 3: Reapply a line of new silicone where the tank panes meet and smooth it in one motion so that it meets both panes. (I just use my finger)

Step 4: Wait for it to cure/dry. Again, no moving the tank.

Step 5: Test it and adjust silicone as needed.

How to Repair a Fish Tank (that currently has aquarium plants/fish in it)

This is infinitely more annoying than repairing an empty tank, but it's still doable. You just follow the same steps outlined above for empty tanks, but with more legwork before and after to keep the tank environment as healthy as you can in the transition and to prevent damage to your home. 

Before the Repairs

First, protect yourself in the situation. If an accident just happened and there is broken glass, proceed slowly so you don't accidentally cut yourself, and don't handle it with your bare hands (seems like common sense, but if you're panicking and your brain tells you to cover the hole and keep a fish from falling out with the leaking do the math). Once you've swept up most of the glass, you can use tape or a damp paper towel to pick up the small pieces you can't see.

If water from a crack has leaked into your electrical components (e.g. a power strip for the heater, light, etc.), don't touch it either until you have rubber gloves and have turned that outlet off unless you won't come in contact with the water when you unplug it. You don't want to shock yourself. You also can't use those again until they've completely dried out or you risk an electrical fire (not so fun fact: aquariums are one of the leading causes of house fires). 

Remove the fish from the tank and place them in a temporary container with as much of the old tank water as you can. On that note, save some of the original water. Keep the filter medium and the substrate/decor wet if you can so you don't lose all your beneficial bacteria and have to cycle your tank all over again. Make sure wherever you put the fish, they can live in it safely for at least a few days. If any of them fell out of the tank or are injured, keep them separate in a hospital tank and tend to them as needed. 

Keep aquarium plants submerged in the original tank water in another tank if you can. If you don't have another tank, you can use wet newspaper to keep them wet.

After the Repairs

Treat your tank like you would a new setup and acclimate your fish slowly. Don't be surprised if your aquarium plants melt back for a while as they adjust to the new environment again. Monitor the tank for leaks, and keep an eye on your water testing. Keeping the beneficial bacteria alive in the filter/gravel and having original water is a huge help, but there will still be some changes in the water chemistry. 

Options for Tanks that Can't Be Fixed No Matter What You Try

If you have a tank that no longer holds water but is still holding together, you can find other uses for it. It could make a great terrarium for non-aquatic pets like geckos, frogs, etc. (just make sure holes and cracks won't let them escape or hurt them with sharp edges). I also like using them as planters or seed starters for terrestrial plants because the glass can make a mini greenhouse and lock in humidity for tropical species. If it's completely shattered, I'll pick it apart and add it to my bin of aquarium scraps to use for any future repairs. 

Final Tips and Tricks

Keep in mind once you've repaired a tank, it's less stable. Have I repaired tanks successfully and had those repairs last for years? Yes - but I still never trust them and am always double checking them. I always keep extra repair stuff and temporary tanks on hand just in case. 

If you ever have to move a repaired tank, handle it with more care to prevent new damage or reopening old damage. Also, watch it more closely than your other tanks. Think of it as looking to buy a used car and finding some accident reports on it. It's cheaper, it's most likely going to be fine since it's been tested, but you're still going to be warier of it and need to know what you're getting into. But, you could get a flashier repaired model for the same price as a less fancy, brand new model. 

Also understand that even though bigger is better with aquariums 99% of the time, this is where it works against you. The bigger the tank, the heavier it is, and the more support it needs. Any damage to that support, even repaired, weakens the tank. Bigger tanks + bigger cracks? No bueno. 

Here are some videos Dustin did on the subject; you know you wanna watch: 

How to Buy a Used Tank and How to Patch a Cracked Tank

How to Fix a Broken Aquarium, "I'm Broke with a Broken Aquarium" 

Good luck with your repairs; may all your plans hold water. ;) 

Tank on!

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