How to Add Oxygen to a Fish Tank (Fast, and for Good)
By Yechiel Kuperman, marine biologist · 49 years keeping fish
Quick answer
Oxygen gets into a fish tank where water meets air, so the fastest way to add more is to move the water: point the filter outlet at the surface, add an air stone, or lower the water level so the filter splashes.
Warm water holds less oxygen than cool water, and a crowded, overfed or dirty tank uses it up faster.
In an emergency such as a power cut, scoop water out with a clean cup and pour it back from a little height every few minutes until the pump runs again.
The numbers
| Water temperature | Most oxygen it can hold | Same in ppm | What it means |
|---|---|---|---|
| 15 °C (59 °F) | 10.1 mg/L[2] | 10.1 ppm | Cool water, like an unheated goldfish tank |
| 20 °C (68 °F) | 9.1 mg/L[2] | 9.1 ppm | A cool room with no heater |
| 25 °C (77 °F) | 8.3 mg/L[2] | 8.3 ppm | A typical tropical tank |
| 28 °C (82 °F) | 7.8 mg/L[2] | 7.8 ppm | The warm end of the tropical range |
| 30 °C (86 °F) | 7.6 mg/L[2] | 7.6 ppm | A heatwave tank: less oxygen, and hungrier fish |
| What most fish need | above 5 mg/L[7] | above 5 ppm | A stocked tank rarely sits at the full maximum |
| Where fish gulp at the surface | roughly 2 mg/L | roughly 2 ppm | Serious distress. Act now |
Do this — step by step
- Point the filter outlet so it ripples the surface of the water.A dimpled surface is doing the work
- Add an air pump with a fine air stone, placed low in the tank.The bubble stream also lifts bottom water up
- Leave a gap between the water and the lid, and never seal the lid airtight.The air above the water needs refreshing too
- Keep the filter flowing at full strength.Rinse clogged media in old tank water only
- Remove uneaten food and dead leaves the same day.Anything rotting uses oxygen
- Add extra aeration during heatwaves, when warm water holds less.
- If the power fails, scoop and pour water back from a little height every few minutes.A battery air pump is cheap insurance
- If fish still gasp with the surface moving, test ammonia and nitrite.Poisoned blood looks like low oxygen
How does oxygen get into aquarium water in the first place?
A fish can't take a breath of air. Every molecule of oxygen it uses has to be dissolved in the water first, then picked up as water flows across its gills. Yechiel's printed guide opens with exactly this, because everything else follows from it.
Oxygen dissolves at the boundary between water and air. In a tank that boundary is the surface, plus the skin of every bubble rising through it. At night, with plants in the dark, nothing else adds any.
Oxygen spreads through still water very slowly. Left alone, the layer at the top stays topped up while the water down by the gravel, where fish swim and leftover food rots, runs low. Moving water carries that tired water up to the surface to refill.
That is the reason Yechiel's rule is about movement. Moving water = oxygen. Still water = trouble. A filter outlet that dimples the surface turns the whole tank over through that boundary, many times an hour.
Air stones help in two ways. Oxygen crosses into the water from each bubble on its way up, and engineering studies of bubble aeration found the bubbles did most of the transfer (DeMoyer et al., 2003). The rising column also drags deep water up to the surface.
Smaller bubbles transfer more, because a cloud of fine bubbles has far more surface than a few big ones. That is the case for a fine air stone over an open tube.
Temperature sets the ceiling. Fresh water at sea level can hold about 9.1 mg/L of oxygen at 20 °C, 8.3 at 25 °C and 7.6 at 30 °C. Warm the tank and the maximum drops, while the fish, whose bodies run at water temperature, burn oxygen faster.
Everything alive in the tank draws on the same budget. Fish breathe, the filter bacteria breathe, and so do the bacteria rotting any uneaten food. That is how overfeeding steals oxygen, which Plecy explains in the feeding guide.
Live plants make oxygen in the light and use it in the dark. In farm ponds, oxygen is lowest just before dawn for this reason. A heavily planted tank still needs its water moving overnight.
Gasping at the surface is the classic sign of low oxygen, but it proves nothing alone. Nitrite poisoning stops blood carrying oxygen, so a fish can gasp in water that is full of it. If the surface is rippling and fish still gulp, test the water.
The Merck Veterinary Manual is blunt about the stakes: "Low dissolved oxygen and high ammonia are the two water quality parameters most likely to directly kill fish."
By the way, out in the ocean…
Dolphin. Famous for intelligence and play — dolphins solve problems and call each other by name with signature whistles. A dolphin comes up to breathe air. Your fish never can: every breath it takes is oxygen already dissolved in the water.
Manta ray. It glides on wing-like fins up to seven metres across — a giant so gentle it eats only tiny drifting plankton. Mantas must keep swimming to push water over their gills. In a tank, the filter's current is what keeps fresh water arriving.
Ozzie the Oxygen Bubble’s Logbook Card
Oxygen bubble · air stone, maximum enthusiasm
Hello! Ozzie here! Straight up from the air stone and heading for the surface at top speed, which is the only speed I have!
Here is what nobody tells you! On my way up I hand oxygen to the water! Then I drag the deep water up behind me so it can grab some more at the top! Two jobs! One bubble!
Warm water holds less of me. Crowded water uses me up. Rotting food eats me alive! So keep the surface moving, keep the food tidy, and please, please do not unplug the pump to make the room quieter! I am supposed to be heard!
What goes wrong with oxygen
Switching the air pump off at night because it hums
Night is when a tank needs it most. The lights go off, plants stop making oxygen and start using it, and every living thing in the water keeps breathing until morning.
If the noise is the problem, a better pump or a folded cloth under it fixes that. Keep it running.
Believing a bubbler in the corner is enough
One small stream of bubbles in a crowded, warm, overfed tank is a sticking plaster. It helps, but it can't keep up with a load that is too big.
Look at the causes as well: how many fish, how warm, how much food. Those set the demand, and aeration only covers the supply.
A glassy, still surface
When the filter outlet sits deep and the surface doesn't move, the tank's one big doorway for oxygen is barely open. An oily film on top can narrow it further.
Raise or angle the outlet until you see the surface dimple. You should be able to spot the movement from across the room.
Letting the filter clog
A filter packed with muck moves a fraction of the water it should, and oxygen drops quietly as the flow fades.
Squeeze the media in a bucket of old tank water when flow slows. Never under the tap.
Treating every gasp as an air problem
If the surface is moving and fish still gasp, more bubbles won't fix it. Test nitrite and ammonia, because both damage breathing in well-aerated water.
Questions we get asked
How can I add oxygen to my fish tank without a pump?
Move the water by hand. Scoop some out with a clean cup and pour it back from a little height so it splashes, every few minutes.
Lower the water level so the filter outlet splashes, and remove any leftover food. For longer outages, a battery-powered air pump is cheap and worth owning.
How do I know if my fish tank has enough oxygen?
Healthy fish breathe steadily and use every level of the tank. Fish crowding the surface, gulping, or breathing fast near the filter outlet suggest oxygen is short.
A dissolved-oxygen test kit or meter gives you a real number. Aim for above 5 mg/L, which is 5 ppm.
Does a filter add oxygen to a fish tank?
Yes, when its outlet ripples or breaks the surface. The filter carries water from the bottom of the tank to the top, where it can take up oxygen from the air.
An outlet buried deep under a glassy surface adds much less. Angle it upward until the surface dimples.
Do fish need an air pump?
Not always. A tank whose filter keeps the surface moving, with sensible stocking and a steady temperature, can hold plenty of oxygen without one.
An air pump earns its place in warm tanks, crowded tanks, planted tanks at night, and any tank where fish are gasping.
Do all fish need oxygen?
Yes. Every fish needs oxygen. Most take it from the water through their gills. A few, such as bettas and gouramis, also gulp air at the surface with a special breathing organ, and corydoras can take in air through the gut.
Even those air-gulpers still use their gills, so every tank needs a moving surface where air meets water.
Do bubbles add oxygen to the water?
Yes. Oxygen crosses into the water from each bubble as it rises, and fine bubbles transfer more than big ones because together they have more surface.
The bubble stream also lifts deep water up to the surface, where it picks up more oxygen from the air. Both effects help your fish.
Why are my fish gasping at the top of the tank?
Low oxygen is the first suspect. Check the filter is running and the surface is moving, and add aeration.
If that doesn't help within an hour, test ammonia and nitrite. Nitrite poisoning stops blood carrying oxygen, so fish can gasp even in well-aerated water.
Does warm water have less oxygen?
Yes. Fresh water at sea level holds about 9.1 mg/L of oxygen at 20 °C (68 °F) but only about 7.6 mg/L at 30 °C (86 °F).
Warmer fish also use oxygen faster, so a heatwave squeezes the budget from both sides. Add aeration when the room gets hot.
Do live plants add oxygen to a fish tank?
In the light, yes, through photosynthesis. In the dark they use oxygen, just like the fish, which is why pond oxygen is lowest just before dawn.
Plants are good for a tank. Keep the water moving overnight anyway, especially in a heavily planted tank.
Sources
- [1] Aquarium Basics, Part 2Kuperman, Y. Aquarium Basics, Part 2 of 9: Oxygen and Water Movement. Yechiel's Fishery Farm care guide (PDF). View source
- [2] YSI oxygen solubility tableYSI (a Xylem brand). Dissolved Oxygen Tables: Calibration and Oxygen Solubility Tables. Technical note XA00055. Fresh water at 760 mmHg. View sourceNote: Values are for fresh water at sea-level pressure. At altitude the maximum is lower. The same figures are produced by the USGS DOTABLES calculator.
- [3] USGS DOTABLESU.S. Geological Survey. DOTABLES: dissolved oxygen solubility tables for a range of temperature, pressure and salinity. View source
- [4] UF/IFAS FA031Francis-Floyd, R., Watson, C., Petty, D. & Pouder, D. B. Ammonia in Aquatic Systems. University of Florida IFAS Extension, FA16/FA031. View sourceNote: The source for keeping dissolved oxygen above 5 mg/L.
- [5] DeMoyer et al. 2003DeMoyer, C. D., Schierholz, E. L., Gulliver, J. S. & Wilhelms, S. C. (2003). Impact of bubble and free surface oxygen transfer on diffused aeration systems. Water Research 37(8), 1890–1904. View sourceNote: Measured in large engineering test tanks, not home aquariums. It supports the direction of the claim (bubbles transfer oxygen directly), not an exact share for a small tank.
- [6] Mississippi State ExtensionMississippi State University Extension Service. Water Quality of Catfish Ponds. View sourceNote: A pond source, used for the night-time oxygen dip caused by plant and algae respiration.
- [7] Merck Vet ManualFrancis-Floyd, R., Yanong, R. P. E. & Petty, B. D. Environmental Diseases of Aquatic Animals in Aquatic Systems. Merck Veterinary Manual. View source
Want the original printed guide?
Download the PDFAbout the author
Yechiel Kuperman
Marine Biologist & Aquaculture Mentor
Yechiel has kept fish since he was four — 49 years with tanks — and 30+ years running farms. He mentors fish farmers internationally.
He writes the Yechiel's Fishery Farm books, in which the biology is real and the fish have opinions.
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