Fishtank Fertilizer FAQ

Posted by Augusta Hosmer on

Read Time: 7 mins

Welcome back to part 146,225,453 of aquarium concepts companies like to make complicated for absolutely no reason!

You're starting a planted tank and you realize, "Oh, hey, my plants look good now, but I bet they'd look amazing if I fertilized them." You Google aquarium fertilizers and are blasted with dozens of products, ads, and advice forums and none of them say the same thing.

Don't worry. Choosing and using aquarium ferts is a lot easier than it looks - and you may not even need them. Here, we'll answer some of the most commonly asked questions about fertilizing your aquatic plants and get you on your way to a better looking aquascape. Check back later for advice on fertilizing ponds!

What even IS fertilizer, and how does it work? 

This is one of those simple questions everyone thinks they know the answer to until they're asked. The most common answer is, "Well...stuff that makes plants grow faster."

It does, but here's why. Fertilizers are not miracle plant growers, but substances that fill in the gaps between what the plant needs and what the plant's environment provides. If a plant's needs are met, it grows better. Simple as that. Another goal of fertilizing is ensuring proper nutrient balance to prevent problems like algae outbreaks. 

For more info on what those needs are, check out part 3 of the Aquarium Science Series on nutrients. But just for a summary, we've provided the image below. 

A chart categorizing plant nutrients as micronutrients, macronutrients, and secondary nutrients.

What forms/types of aquarium fertilizer are there?

There are four main forms: liquid ferts, dry ferts/salts, root tabs, and substrates/substrate additives. You can also technically count CO2 supplementation as fertilizing, but it's a bit different. So we'll have another blog dedicated entirely to CO2 in the near future - be on the lookout!

  • Liquid Fertilizers - contain concentrated nutrients in a liquid form (imagine that) you can pour directly into your water column. Dose usually measured in milliliters (mL)
  • Root Tabs - small capsules or tablets (some of them seriously look like you picked them up from the pharmacy) placed directly in the substrate near the plant's roots
  • Dry Fertilizers - dry granules, powders, or salts usually dosed in grams (g) and dissolved in water. Most pond fertilizers come in granular or powdered form
  • Substrates/Substrate Additives - some substrates (like Fluorite or Stratum) come already dosed with nutrients. There are also several additives you can mix into the substrate to supplement specific nutrients (crushed coral for calcium, for example).
A collage of 3 common fertilizer methods. Top image: green liquid fertilizer; bottom left image: root tabs; bottom right: crushed coral

Within these categories, there are two types of fertlizer: mixed (AKA complete, balanced, or compound) or single-nutrient (AKA straight). Mixed fertilizers contain multiple nutrients in varying ratios, while straight fertilizers contain concentrated amounts of a single nutrient (like iron supplements). 

You can also break fertilizers down into processed or organic ferts. Most of the aquarium market is processed fertilizer

Do I really need to fertilize? 

The short answer: depends on your setup. If you're growing easy, hard-to-kill plants with low nutrient demand in a fish tank, it's not critical. The plants will grow more quickly and be more vibrant with fertilizer, but they can still look fine without it. So, for a simple setup: recommended, not required at first. Think surviving vs. thriving.

That said, unless your tap water is providing enough, this no-fertilizing system will only last until micronutrients in the system are depleted. Fish produce several primary nutrients plants use (mainly nitrogen), but not micronutrients like iron. 

However, if you have a solely planted tank with no fish to provide nutrients via waste, a heavy plant load, heavy root feeders like sword plants, no substrate, or other plants with higher nutrient demands, you will need ferts if you want to have a chance. This does not mean more nutrients = plants will automatically grow better or redder, but we'll get into dosing further down. 

Plants you can grow without fertilizers, but may grow more slowly: Java Ferns (Microsorum); AnubiasWater Wisteria (Hygrophila difformis); Bacopa; Banana Plants (Nymphoides aquatica); most mosses (Java moss, flame moss, etc.); Anacharis; duckweed
Plants you may need to supplement: Sword Plants (Echinodorus); Ludwigia; Rotala

When should I fertilize my aquarium plants?

This is the question that confuses folks, but it's deceptively simple.

Plants don't eat food the way we do, but their nutrient usage follows a similar pattern. Who needs more nutrients: a professional weight lifter, or an accountant (if you are a professional weight lifting accountant I apologize for the metaphor and I think you're amazing)? Ever heard someone tell a kid, "Eat up, you're growing?" Same concept. Plants need more nutrients when they're doing more: in times of growth and demand.

So, the best time to fertilize is just before a plant's nutrient demands will be high. 

Dosing as soon as the plants are transplanted is a common practice that works well. If you're growing plants from seeds, fertilizing when you seed can give them a boost. If you have floating plants that bloom, dosing before bloom helps support the plant through the extra energy expenditure. Plants will also need more nutrients during breeding or propagation. Finally, regular dosing as your plants grow, especially with rapid growers (dosing depends on your setup; check out the next question for more info), can keep your planted setup looking lush and green. 

How do I know how much fertilizer to dose, and how do I apply it?

*As an overall note, it's better to not fertilize than to fertilize incorrectly*

The answer to this question depends on the form of fertilizer you're using and a slew of other factors, like how heavily your tank is planted, your plant species, your water chemistry, what your goals are, etc. So you'll need to experiment with your individual tank.

Ideally, you'd have a way to measure the nutrients in the tank before adding plants and monitor their levels as the plants grow to know exactly how much to supplement, but this is time-consuming, often costly, and overwhelming to a new hobbyist.

Don't get me wrong: there are tests out there that let you measure the levels for every single nutrient, but to get regular and accurate readings, it's pricey, and typically involves sending off a water sample to a lab. Not something I personally want to worry about all the time, but to each their own.  

The fertilizer label will have information on what nutrients it contains, how to apply it, and how much it provides with a certain amount. Be sure to read labels thoroughly, follow directions, and monitor how your setup responds. 

There are two main schools of thought for how much/how often to dose fertilizer. One is frequently dosing the water with nutrients and managing the tank to ensure this doesn't cause algae and other problems to explode, often called the Estimated Index (EI method). 

The second is only supplementing nutrients the tank environment (mainly the substrate) isn't providing, encouraging a rich substrate. Both methods have pros and cons.

A bottle of aquarium iron supplement

Applying Liquid Fertilizers

Liquid ferts are poured directly into the water column. Typically, the bottle will direct you to use the cap as your measuring tool. Seachem Flourish, for example, breaks dosages into 5mL segments and has a 5mL cap. I've personally always found using plastic syringes or pipettes easier and more accurate (especially if I'm dealing with species sensitive to environment changes or with a weird tank volume) but the cap does work. 

Some also use a pump method where each pump contains a certain amount of nutrients. This typically isn't as accurate either, but, like the cap, is still a viable choice. 

I like to add liquid ferts where they will flow easily to the rest of the tank, so near the filter output or a similar location, but you don't necessarily have to. 

Applying Root Tabs

Root tabs, as the name suggests, are pushed into the substrate, and are great for heavy root feeders (understand a large root system doesn't necessarily mean it's a root feeder, by the way). Contrary to popular belief, you don't have to concentrate the tabs around the roots. Insert tabs at regular intervals throughout the substrate according to label instructions. 

Also, it's not harmful if the root tab floats out of the substrate and dissolves in the water column (It's a common problem), though it does defeat the point of the root tab.

Is aquarium fertilizer safe for shrimp, invertebrates, amphibians, turtles, sensitive fish species, etc.?

We can't answer this for 100% of the products on the market, but generally speaking, yes, you can safely fertilize a planted tank with shrimp, snails, frogs, etc. as long as you do it properly. Just like with anything else you put in your tank, be sure to read labels, and triple check your dosing instructions.

The main concern folks with shrimp tanks have with ferts is copper and shrimp's sensitivity to change. But if you're following instructions and doing water changes, the odds of problems are slim. That said, there are fertilizers made specifically for shrimp and other invertebrates if you're concerned. You can also try dosing with less than the recommended amount the first application and gradually increasing it over time while monitoring the animals' responses to be extra safe. 

As an added note, be very cautious about using fertilizers designed for terrestrial plants, because they often can be harmful to aquatic organisms. 

Got questions? Blog post ideas? Hit us up, and tank on!

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