Enough Gas Exchange?

[intarsiabox]

Would a tank with dual overflows get enough gas exchange from the overflows if I were to cover the top of the aquarium?

[Myka]

Are you running a skimmer? If you have a skimmer, you can usually cover the tank without any issues provided the skimmer is getting a good amount of fresh air. Simply running the air injection line to the outside of the stand will greatly improve fresh air exchange.

[Ryan]

I have a cover for my aquarium but I leave a little gap for gas exchange, feeding hole and to stick my fingers in there to lift the glass if I want to get into the tank.

I have a skimmer on my tank and havent noticed any problems.

[Oxymoron]

I would imagine that if you pointed your powerheads to create some surface agitation. As well as using a skimmer in the sump would provide enough gas exchange.

[Ryan]

Rippling the surface does nothing for gas exchange, there has to be a break in the water for proper gas exchange.

[intarsiabox]

I only have an Aqua C Remora at the moment so I don't know if this gives me a lot of O2 into the water. The sump itself would stay open top.

[Myka]

The Remora should do a fine job. It uses a spray injection for the bubbles, and it creates quite a few. I shouldn't think you would have any issues. If you do, blow a small fan on the skimmer so it gets more fresh air.

[Oxymoron]

Rippling the surface does nothing for gas exchange, there has to be a break in the water for proper gas exchange.

Correct me if I'm wrong but I was under the impression that surface agitation created by powerheads helps break up the film on top of the water for filter's to pick up.
Breaking this film in turn creates a larger surface area for gas exchange to occur.

[Ryan]

Yes but there has to be a break in the tension of water for the air to mix into the water. This is why airstones work so well. There are so many bubble breaking the surface of the water helping with gas exchange.

One question that hasnt been asked is what stocking levels do you have in the tank? How big is the tank?

[Oxymoron]

After reading your first post I spent some time researching the matter.
From what I've found, the general consensus seemed to be that airstones are one of the poorest forms of gas exchange as the surface area of the tiny bubbles are so small.

[Ryan]

There is going to be more surface area in 1000 1ml air bubble there there is on 1 1000ml air bubble. Surface area is what you want with oxygen exchange. Why do you think skimmers have millions of little bubbles instead of a few big bubbles? Because there is more surface area.

[Oxymoron]

I believe that airstones in a reef tank are unnecessary and not needed as long powerheads are positioned to continuously roll water from the bottom of the tank to the top. Your dual overflows and the use of a skimmer should be more than enough. Airstones can also introduce micro bubbles in the display which are unhealthy for the fish. And there's also a build up of salt creep from the spray they create. Hope this helps. Others may have other opinions as this is a topic that has been argued about many times lol.

[Myka]

Airstones rarely have a place in a sw tank, but they can make a big difference in raising pH by off gasing CO2. :D

Both Ryan and Oxymoron are right. The rippling of the water surface does break the layer of greasy organics on the surface, however if you have a sump with an overflow the surface grease should not be an issue. An airstone does increase O2 in the tank (the efficiency of this varies with the quality of the airstone and how fine the bubbles are), but if you are running a skimmer (that fits the size and bioload of the tank well) a skimmer is essentially a giant airstone when it comes to O2 and CO2 gas exchange. If you are using cheapie $2 airstones you aren't going to produce much O2 exchange. Buy some super fine wood or glass airstones, and the results will be much greater.

[Snaz]

Airstones can also introduce micro bubbles in the display which are unhealthy for the fish.

I think the accepted consensus on this point is that it is false. Micro bubbles occur in nature, crashing waves on a reef, with no ill effect.
http://www.reefkeeping.com/issues/2004-01/eb/index.php

[Myka]

The fish don't care about micro bubbles, but many corals will be irritated by them.

[Ryan]

Thats why you would put them in the 1st chamber of your sump so the baffles take them out.

Nowhere did I say to put them in a display tank but no "rolling" the surface will not help build O2 levels it will just help the water move around. You have to break the surface to help with 02.

Your skimmer, dual overflows, the water entering the sump as well as the water going over your baffles all helps in oxygenating the water.

[naesco]

By placing a power head near the surface of the water in your display tank you are accomplishing 2 things.
1. You are mixing 'more oxygenated water with less oxygenated water which is found lower in the tank.
2. If your power head is creating a ripple effect on the water surface, the water is being oxygenated in the same way that a moving steam in nature is as opposed to a sitting pond for example.

No scientist here but it makes common sense, doesn't it?

[intarsiabox]

Yes but there has to be a break in the tension of water for the air to mix into the water. This is why airstones work so well. There are so many bubble breaking the surface of the water helping with gas exchange.

One question that hasnt been asked is what stocking levels do you have in the tank? How big is the tank?

My tank has a 30" x 20" surface area, no stocking yet as I just finished leak testing all the plumbing and am in the process of refilling with RO/DI water. The tank is about 50g and about 15g more in the sump/refugium. It sounds like I should be okay with a glass top. I have never used one before as I've always used MH but for this tank I bought a T5HO fixture so heat shouldn't be as big an issue.

[o.c.d.]

The primary change in oxygen change ,happens from photosynthesis. It is the most effective and fastest way to get O2 into the water column. Skimmer, and water movement do help but not as significant as photosynthesis. The best way to have more O2 in your system is to have longer light spans, and reverse lighting in refugiums.