Surface agitation amount necessary for gas exchange?

swiss1939 · Sep 7, 2025

I've been wondering exactly how much surface agitation is needed for proper gas exchange for a little over a month. Ever since I set up my new tank using a canister filter, its been on my mind. I set this canister filter up because I wanted a cheap/quick tank set up without a sump, and I didn't want to build a AIO with the cheap petco tank I bought for this purpose. I also love the fact that the running tank would be silent without an overflow. But ever since I put my first fish in, I've been cognisent that the lack of overflow with the canister filter might be causing low oxygen issues in my tank. I've set up my power head aimed at the surface to create surface agitation, but it does not really "break" the surface, it just creates ripples in the surface. I also have an aeration valve on the outlet where water is coming back from the canister filter, which I mostly keep off because it adds noise when lots of bubbles are generated. I will turn the bubble aerator on full blast for a few minutes every few days just out of that concern going on in the back of my mind.

But I have been busy with work the last week and a half and completely forgot to run the aerator for a week or so. This morning I woke up and my clownfish was swimming in the corner where she likes to swim between the glass and the canister filter's inlet tube, but swimming nose down vertical. So I fed her, and she was swimming fine while eating, but then went straight back to swimming vertically in the corner again. I couldn't figure out if she was just choosing to do this kinda like she wasn't fully awake yet and was "sleepwalking", or if there was something underlying going on with the oxygen level in tank (I had just tested Nitrate and Phosphate two days earlier so I knew those levels are good). I ended up leaving the bubble aerator on full blast for half an hour just to see if that would help. The clownfish was way more active and swimming normal after that.

So now I'm seriously thinking about the amount of surface agitation needed for proper gas exchange when before I was just curiously thinking about it.

What are peoples understanding/knowledge/thoughts about what the true definition of "surface agitation" is regarding ensuring proper oxygen exchange in a tank? I've only ever had a HOB/AIO or skimmer running on tanks before which all have no issues with providing enough gas exchange/oxygenating the water. This is my first time running only a canister with both ends of the filter below the surface of the tank.

When people say "surface agitation" does that mean just rippling of the surface with no breaking of the surface? Or does that mean the surface has to break from something like water pouring down through the surface, or from air bubbles popping at the surface from a bubbler?

Here is a video of my power head surface agitation when the aerator on my canister outlet is turned completely off:

Ripples are created, but surface is not "agitated" enough to break the surface and create bubbles.

And here is the aerator on the outlet of my canister filter when fully open:

I can easily run the aerator cracked slightly open just to add a few bubbles, which would be minimal noise, compared to wide open. But the question I'm still debating is on what that true definition of surface agitation is when looking for proper gas exchange?

Thoughts?

Dan_P · Sep 7, 2025

swiss1939 said:
I've been wondering exactly how much surface agitation is needed for proper gas exchange for a little over a month. Ever since I set up my new tank using a canister filter, its been on my mind. I set this canister filter up because I wanted a cheap/quick tank set up without a sump, and I didn't want to build a AIO with the cheap petco tank I bought for this purpose. I also love the fact that the running tank would be silent without an overflow. But ever since I put my first fish in, I've been cognisent that the lack of overflow with the canister filter might be causing low oxygen issues in my tank. I've set up my power head aimed at the surface to create surface agitation, but it does not really "break" the surface, it just creates ripples in the surface. I also have an aeration valve on the outlet where water is coming back from the canister filter, which I mostly keep off because it adds noise when lots of bubbles are generated. I will turn the bubble aerator on full blast for a few minutes every few days just out of that concern going on in the back of my mind.

But I have been busy with work the last week and a half and completely forgot to run the aerator for a week or so. This morning I woke up and my clownfish was swimming in the corner where she likes to swim between the glass and the canister filter's inlet tube, but swimming nose down vertical. So I fed her, and she was swimming fine while eating, but then went straight back to swimming vertically in the corner again. I couldn't figure out if she was just choosing to do this kinda like she wasn't fully awake yet and was "sleepwalking", or if there was something underlying going on with the oxygen level in tank (I had just tested Nitrate and Phosphate two days earlier so I knew those levels are good). I ended up leaving the bubble aerator on full blast for half an hour just to see if that would help. The clownfish was way more active and swimming normal after that.

So now I'm seriously thinking about the amount of surface agitation needed for proper gas exchange when before I was just curiously thinking about it.

What are peoples understanding/knowledge/thoughts about what the true definition of "surface agitation" is regarding ensuring proper oxygen exchange in a tank? I've only ever had a HOB/AIO or skimmer running on tanks before which all have no issues with providing enough gas exchange/oxygenating the water. This is my first time running only a canister with both ends of the filter below the surface of the tank.

When people say "surface agitation" does that mean just rippling of the surface with no breaking of the surface? Or does that mean the surface has to break from something like water pouring down through the surface, or from air bubbles popping at the surface from a bubbler?

Here is a video of my power head surface agitation when the aerator on my canister outlet is turned completely off:

Ripples are created, but surface is not "agitated" enough to break the surface and create bubbles.

And here is the aerator on the outlet of my canister filter when fully open:

I can easily run the aerator cracked slightly open just to add a few bubbles, which would be minimal noise, compared to wide open. But the question I'm still debating is on what that true definition of surface agitation is when looking for proper gas exchange?

Thoughts?

Add another power head on the opposite side of the aquarium if you have concerns

Pistondog · Sep 7, 2025

If we could easily measure dissolved gas levels in our tanks, we might be more concerned. We have a waterbox aio and run an air stone to augment gas exchange. I think ripples along the surface is what we shoot for.

Mr. Mojo Rising · Sep 7, 2025

I like to see a few bubbles enter the water from the agitation, I accomplish this with 2 powerheads pointed at each aimed upwards towards the surface. I would suggest that you need more surface agitation.

swiss1939 · Sep 7, 2025

I am now leaving the aerator on to provide some bubbles constantly. Not full blown crazy agitation, but enough where it would be similar to the same from a HOB overflowing back into the tank.

1ocean · Sep 7, 2025

clown fish typically like to sleep and or hang out in place they feel comfortable...All my clowns do the same thing for a while then come out for a while...
If your clowns needed O2 they would be breathing hard so would all your fish..

Randy Holmes-Farley · Sep 7, 2025

I’m not a fan of adding bubbles into a display due to salt spray at the top when they pop, encrusting salt on lights and other things just above the water.

In terms of gas exchange, almost no reef tank is complete. If it were, pH would not change day to night, but would only change as the CO2 in the room air changed. If one does not measure O2, then the size of the pH swing is the best way to measure gas exchange.

It’s thus a matter of how much equilibration of O2 and CO2 are necessary for creatures to thrive. Factors such as skimmers also at a big role in gas exchange.

PapaFishRocks · Sep 7, 2025

I run a canister on my 90g.

Could never get my pH above 7.9.
Increased surface agitation, and just ended up with salt spray everywhere.

During my last water change, I lowered the water level 1/2”. Just to see what would happen.

pH at 8.1-8.2. Salt spray almost gone. And the corals seem happier too.

swiss1939 · Sep 7, 2025

Randy Holmes-Farley said:
I’m not a fan of adding bubbles into a display due to salt spray at the top when they pop, encrusting salt on lights and other things just above the water.

In terms of gas exchange, almost no reef tank is complete. If it were, pH would not change day to night, but would only change as the CO2 in the room air changed. If one does not measure O2, then the size of the pH swing is the best way to measure gas exchange.

It’s thus a matter of how much equilibration of O2 and CO2 are necessary for creatures to thrive. Factors such as skimmers also at a big role in gas exchange.

Im also not a fan of bubbles due to the salt spray/encrusting on the lid/etc. Which is why I was trying not to use the aerator. I've got it set pretty low now, just to add some minimal bubbles to help.

swiss1939 · Sep 7, 2025

PapaFishRocks said:
I run a canister on my 90g.

Could never get my pH above 7.9.
Increased surface agitation, and just ended up with salt spray everywhere.

During my last water change, I lowered the water level 1/2”. Just to see what would happen.

pH at 8.1-8.2. Salt spray almost gone. And the corals seem happier too.

Curious how lowering the water level helped you increase pH? I understand lowering the water level would reduce salt spray just by distance, but I'm not following it's effects on pH.

PapaFishRocks · Sep 7, 2025

swiss1939 said:
Curious how lowering the water level helped you increase pH? I understand lowering the water level would reduce salt spray just by distance, but I'm not following it's effects on pH.

Gas exchange. I didn’t move my gyres. So the are now closer to the surface, giving me more surface agitation without all the salt spray and not introducing a bunch of micro bubbles.

swiss1939 · Sep 7, 2025

Again, how? Is it because your power heads were closer to the surface of the water, hence agitation the water surface more? Just lowering the water level does not increase gas exchange on its own.

Randy Holmes-Farley · Sep 7, 2025

swiss1939 said:
Again, how? Is it because your power heads were closer to the surface of the water, hence agitation the water surface more? Just lowering the water level does not increase gas exchange on its own.

That would be the reason, unless something else changes.

Randy Holmes-Farley · Sep 7, 2025

I’ll just add that more tank top aeration does not necessarily raise pH. It may lower it despite increasing gas exchange.

swiss1939 · Sep 7, 2025

My pH is consistently 7.9. I just removed a glassfull of water and lowered the water level about quarter to half an inch. Also repositioned my power head closer to the surface still aimed at the surface... enough that now there is a tiny whirlpool suction at the back of the power head when it randomly blows at it's strongest. This sucks some air down into the powerhead and creates a quick burst of bubbles randomly. Hopefully this all helps. I'll keep an eye on it for the next week and recheck the pH to see if that had any affect.

Randy Holmes-Farley · Sep 7, 2025

swiss1939 said:
My pH is consistently 7.9. I just removed a glassfull of water and lowered the water level about quarter to half an inch. Also repositioned my power head closer to the surface still aimed at the surface... enough that now there is a tiny whirlpool suction at the back of the power head when it randomly blows at it's strongest. This sucks some air down into the powerhead and creates a quick burst of bubbles randomly. Hopefully this all helps. I'll keep an eye on it for the next week and recheck the pH to see if that had any affect.

Hopefully the effect is not a downward shift due to high CO2 in your home air.