FAQ's and troubleshooting for aquarium air pumps, air stones, oxygen-producing plants, etc.
Read Time: ~10 mins
What is dissolved oxygen, anyway, and why do aquarium people keep talking about it?
Brace yourselves for a super technical definition.
Dissolved oxygen (DO)...is the amount of atmospheric oxygen dissolved in the water. It's mostly introduced through water surface movement, so a white water rapid will typically have more dissolved oxygen than a still swamp.
Why does dissolved oxygen matter in aquariums?
Simply put, because your animals and plants need to breathe. It's a common aquarium myth that fish pull oxygen from water molecules when in reality, they utilize the same atmospheric oxygen we do. If there isn't enough oxygen, your fish will experience increased stress, increased disease risk, or even potential death, if DO levels are dangerously low.
If you've got a planted tank, your plants need oxygen for their life cycles, too. And if you're growing plants hydroponically or aquaponically, having enough DO is crucial to prevent rot and other health problems.
What are signs my tank doesn't have enough dissolved oxygen?
One of the telltale signs of inadequate oxygen (hypoxia) in fish is glaringly obvious: gasping for air and "panting," in their own way.
Fish breathe through their gills, mostly (there are some labyrinth organ fish that can also breathe air, but we'll get to them later), so if you see a lot of rapid gill movement or really red gills, that should set off alarm bells. Sometimes, they'll quickly open and close their mouths and gasp for air.
Fish in hypoxic water will also hang out near the surface where there's more oxygen, so if your fish are suddenly swimming around the top of the tank more, that could be a symptom.
One other weird thing I've noticed in tanks without enough oxygen (ponds, too) is they often smell bad. That's not saying if your tank smells it's hypoxic, but it is worth mentioning.
These next two aren't exactly signs of hypoxia, but are good indicators you need to be particularly concerned about providing enough DO: tank dimensions and water movement.
Like I said above, DO is higher in turbulent water with more surface agitation, so if your tank water is totally stagnant, that's hypoxia waiting to happen.
You also need to be more concerned if your tank is deeper and taller than it is wide. Shallower tanks have a greater surface area for gas exchange and diffusion than vertical tanks, and it can be more difficult to circulate gas to lower parts of tall tanks.
If you want to be 100% sure your water is hypoxic, you can also just test dissolved oxygen levels, but usually, it's obvious enough you don't have to. If you want to test it, there are electrical monitoring systems or testing kits you can buy.
How much dissolved oxygen do fish need?
It varies a bit, but most experts (Extensions, universities, and Agriculture/Fish and Wildlife departments) recommend 5-6 mg DO per liter of water (AKA 5-6 ppm). Below 2ppm is deadly.
Current research also indicates cold water fish need more dissolved oxygen than warm water fish due to warm water species' greater ability to unload oxygen. That said, warm water doesn't hold onto DO as easily as cold water, so you need to strive for balance.
Is it possible to have too much oxygen in a fish tank?
It's difficult to hyperoxygenate an aquarium with most setups with regard to DO levels, but yes, it is possible, and it's also a problem. Too much oxygen can lead to health problems or increased cortisol (stress) levels in fish.
It's also possible to over aerate in the sense you create water that's TOO turbulent and your fish are basically living in 24/7 Splash Mountain. You need to find the right water flow balance for your tank, especially if you have fish with longer fins or species who prefer quiet water.
Also, if you're breeding fish who lay eggs on the surface, too much surface agitation can really mess up your plans.
If I have a betta or other fish with a labyrinth organ, do I really need to worry about oxygen since they can just breathe air?
The short answer is yes. The long answer is bettas (and other gouramis - same family) live in still water with lower dissolved oxygen in the wild, so they evolved what's called a labyrinth organ that allows them to breathe air in addition to absorbing oxygen from their gills. So they technically have the ability to survive low DO and are more resistant to hypoxia than other species, but they are less stressed when there's sufficient DO in the water.
How do I increase DO in my aquarium?
There are tons of options for boosting your tank's DO. The most common ways are mechanical devices like bubblers/air stones, spray bars, etc.; types of filtration; and live aquarium plants.
Does my tank really need an air pump?
It depends. If you have a power filter, trickle filter, etc. that provides decent surface agitation, you are probably okay. That said, I don't run any tanks without airstones. One, because I like to have as much oxygen circulation as I can and every little bit helps, and two, because that way, if either fails, I at least have something giving my fish DO.
Your fish won't die without an air stone, usually, but I find my tanks are happier with them. Plus, they're cheap, so why wouldn't I?
What do I do for dissolved oxygen if the power goes out?
A lot of people assume your biggest aquarium concerns in a power outage are lights and heaters, but the most critical is actually making sure you don't let your water get too still. Fortunately, there are several cheap, easy ways to handle this.
My go to will always be either a battery-operated air pump or a USB air pump. You can also periodically agitate the water surface manually to simulate the action of a power filter. Never let your tank just sit there for several days. I like to have a battery-powered pump operating in addition to my other aeration just in case the power goes out when I'm not home, too, but you don't absolutely have to do this. Also a good idea to have check valves on your tubing (more on that below).
For more info on dealing with aquariums in emergency scenarios, check out one of our other blog posts on it!
How do my other water parameters affect DO?
Nothing ever changes in isolation in an aquarium or pond environment. Temperature, time of day, algae/plant growth, chemical changes, etc. all play key roles.
- Temperature - this is going to be oversimplified, but basically, warmer water doesn't hold onto oxygen as easily as colder water.
- Time of day - you'll see this especially in ponds, but if you have lights on a regular cycle in a tank, you can observe it on a smaller scale, too. DO levels increase throughout the day, are highest at around dusk (or right before you shut off the lights in a tank), and then gradually decrease throughout the evening. This is helpful to you as a hobbyist because you can predict when DO levels will be at their lowest - if your fish are fine during those hours, you are likely providing enough DO.
- Plants/Algae - A heavy plant load significantly increases your DO. However, one of the most common causes of catastrophic hypoxia is sudden plant/algae death or removal. That's why you'll hear horror stories of someone's cleaning up an algae bloom in their tank and accidentally sending their fish into shock, or having a massive fish kill because of algae death in a pond. Think about it: algae photosynthesizes too. If you take away a significant source of oxygen rapidly, you're going to have some undesirable, fast changes in your water chemistry.
A line graph from the University of Florida Institute of Food and Agricultural Sciences (author Ruth Francis-Floyd) showing how dissolved oxygen fluctuates throughout the day in a pond. This pattern is called a diurnal oxygen cycle.
How do I make an air pump less noisy?
Fortunately, manufacturers are starting to make them quieter and building in cushioning, rubber feet, etc. But if you don't have a newer one, you can try keeping the pump on foam, cloth, etc. I've seen some folks use kitchen sponges. You can also just hang it up so it isn't rattling against your glass as long as it doesn't put strain on cords or lines.
Make sure it's sitting evenly, too, because I have a quieter one and if I knock it a little off kilter, it yells at me until I fix it on more solid ground.
If it's aggressively noisy (like "what the hell, is this an airpump or a chainsaw" kind of noisy) there could just be a mechanical problem that either needs fixing or replacing.
Do I need a check valve for my air pump?
Generally yes, but it depends. If you're not sure what a check valve is, it's a little accessory that prevents water from being sucked out of the tank if your pump turns off for some reason. You set up your pump, attach tubing to the pump, cut the tubing in the middle, attach the check valve to the two halves of tubing, then attach the other end of tubing to your airstone or bubbler. Make sure your check valve is facing the right way (sincerely, an idiot who flipped it one time).
If your air pump is above surface level, it would be physically difficult to siphon water out of the tank, so you may be able to get away without a check valve if your pump is situated that way.
Given check valves cost a few bucks at most, though, I use them just to be safe even though most of my air pumps are above surface level. Some air pumps have check valves built in, so give your manual a look.
"My airstone isn't bubbling as much anymore and/or looks dirty."
Since airstones are full of pores to release air, it's easy for them to get clogged and stop working. You can clean them out with diluted white vinegar and water. Also pretty easy to soak or boil debris out of them. I usually give them a quick clean when I'm doing water changes to make them last a little longer, but they're so cheap you can just replace them instead of cleaning them.
When to replace aquarium air lines
Airlines tend to last a long time depending on use and setup. I look at replacement when the tubing gets really discolored, stiff, and brittle. Don't worry if you find a crusty, white substance on them you can scrape off (check out the pic below) because that's likely just harmless limescale buildup from harder water.
If you're not 100% sure if you should replace and you're worried, it doesn't hurt to just go ahead and do it because the tubing is so cheap. I'm just a "use it til it breaks" kinda person.
Is it true you can't run airstones and CO2 at the same time because you'll lose all your CO2 to surface agitation?
Nah, this is a common aquascaping myth. I mean, sure. You're going to lose SOME carbon dioxide with surface agitation, but it's not as simple as "Oh, I'm wasting dosed CO2 if I run it with an air pump." You actually want a good balance of CO2 injection and surface gas exchange if you're supplementing CO2 for your plants, because you don't want CO2 to just constantly build in your tank until it reaches dangerous levels.
What are some of the best aquarium plant species for providing extra oxygen?
All plants will add DO to your water since oxygen is an end product of photosynthesis, but there is research to suggest some do it better than others. Just to name a few:
- Hornwort (AKA coonstail; Ceratophyllum)
- Banana Plants (Nymphoides aquatica)
- Vallisneria americana (AKA Giant Jungle Val)
Do plants suck oxygen out of the tank at night?
Not in a way that's concerning for you.
Plants do use oxygen to respire, but not in amounts that would lead to hypoxia for your fish at night unless you have basically zero aeration aside from plants or have a severely overstocked tank.
If you have a heavily planted tank with a lot of floating plants, plants that block the surface of the water and inhibit atmospheric oxygen exchange, or an aquascape that inhibits gas circulation, this could lead to DO problems. But it's not because plants "steal" oxygen at night.
Got more oxygen questions or another cool idea for a blog post? Send 'em in, and tank on!
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