Surface agitation vs under current and aquarium oxygen content

[batang_mcdo]

i just set up a new tank, my filtration is a canister and an internal overflow system.
it provides some surface agitation. then i have a wavemaker pointed downwards towards the bottom of the tank as I'm planning to move my plecos into this tank too.

just curious, will a wavemaker pointed downwards help with the tank's oxygen level?
or does it have to be pointed upwards to break the surface?

 

[giar]

From what i know, As long as you see some water movement, then there is an oxygen exchange.

 

[Drstrangelove]

Basically, the way gases (o2, co2, etc.) and water work is they reach equilibrium through gas exchange (i.e., diffusion.) If the water.... at the surface ... has less oxygen than is in the surrounding air, then the oxygen in the air enters the water. Carbon dioxide (co2) does the same.

Exchange with the atmosphere is only at the surface. So "breaking" the surface is not as critical as bringing volumes of oxygen deficient water from the bottom of the tank to the top. Many portions of lakes and oceans are oxygen deficient below the surface because the water never comes up from 30-100 feet down.

And using oxygen or air to drive air stones has minimal effect. People have used nitrogen-driven stone diffusers and gotten the same improvement of oxygen in tanks that were otherwise deficient because anything that brings the water up to the surface will work.

This also means that covering the surface of the water (with for example a sheet of plastic) can end up killing the fish due to suffocation if the surface is the only place water interacts with the air.

 

[DN328]

[QUOTE="This also means that covering the surface of the water (with for example a sheet of plastic) can end up killing the fish due to suffocation if the surface is the only place water interacts with the air.[/QUOTE]

Can you elaborate? What about tanks with fully enclosed lids? Thanks.

 

[eddiegunks]

How could airstones not add o2? They are tiny air bubbles.....no?

 

[Galantspeedz]

How could airstones not add o2? They are tiny air bubbles.....no?

my understanding is the air bubbles floats to the surface and capture oxygen or creates surface agitation that captures oxyegn.... the air bubbles itself are not filled with oxygen

i.e when packing fish, you need to use oxygen to fill the bag rather than air from air stone.

 

[Drstrangelove]

This also means that covering the surface of the water (with for example a sheet of plastic) can end up killing the fish due to suffocation if the surface is the only place water interacts with the air.

Can you elaborate? What about tanks with fully enclosed lids? Thanks.

This relates to literally taking something (a plastic sheet for example) and laying it across the surface of the water. A thin film of oil would do the same.

A "fully enclosed" lid, despite the fact that it is precisely what everyone (including me) calls it, isn't stopping air, light, water, or sound from penetrating the lid through gaps throughout the structure.

We all often use the term "as if" it's fully protecting and encapsulating the fish, but all it's really doing (and not too well sometimes) is stopping fish from jumping out from, or curious people from dropping things in to, the tank.

It's somewhat like a fence that "fully encloses" a yard or horses.

 

[Drstrangelove]

How could airstones not add o2? They are tiny air bubbles.....no?

The total surface area of the bubbles is small compared to the total surface area of the tank. Ceteris paribus, it does add a small amount of oxygen to that immediate area in the tank around the bubbles. But water rises to the surface. This is the important value of the bubbles: to cause oxygen deficient water to rise up to the surface.

People have used air pumps driven by pure nitrogen and gotten the same increase in oxygen levels in the water as from using oxygen.

The reason is because the water reaches a saturation point when it's at the same level as the atmosphere. Pumping in oxygen doesn't make tank water more oxygen filled since as soon as the water reaches the surface it releases oxygen to the air since at that point the air is now "oxygen deficient" compared to the water.

The only way to force more oxygen into the water above it's saturation point, at any given temperature and altitude, is to force more oxygen into the air. That is, hermetically seal the room, and increase o2 content of the air in the room.

 

[DN328]

This relates to literally taking something (a plastic sheet for example) and laying it across the surface of the water. A thin film of oil would do the same.

A "fully enclosed" lid, despite the fact that it is precisely what everyone (including me) calls it, isn't stopping air, light, water, or sound from penetrating the lid through gaps throughout the structure.

We all often use the term "as if" it's fully protecting and encapsulating the fish, but all it's really doing (and not too well sometimes) is stopping fish from jumping out from, or curious people from dropping things in to, the tank.

It's somewhat like a fence that "fully encloses" a yard or horses.

Thanks. True, enclosed rather than being "air tight".