water change %

[piercho]

In reference to water changes mentioned in your July Reefkeeping column:

I drew (what I think are)different conclusions from Dr. Ron's articles on the percentage of water that should be exchanged. To jump straight to it, I think that an effective water exchange will be 50% of system volume or more. My rationalization for this is simple, if there exists a gross excess or deficiency in the water chemistry - and Dr Ron's data seems to indicate that this is often the case - it takes a very large water change for significant dilution. Taking the tank "back to baseline", as I think that you put it. Doing a big water change is a pain, but I figure that doing a 50% change about every other month is less (total) work than doing a 5% change every week. My few coral (all stonies) and my anemone all seem to weather big water changes well.

Anyway, 2 questions. 1)Do you think my theory that one big dilution is better than several small dilutions holds any water? 2)Tribal knowledge seems to be that big water changes are hazardous to "SPS" coral and Tridacnids. Assuming that delta SG < .001 and delta T < 4F during the exchange, is this true?

An additional small point/question on another topic in your article. In my opinion, venturi skimmers may be providing a significant amount of aeration, especially for tanks using "hang-on" equipment and no sump. My tank pH runs high so I choose to let my skimmer run continuously, hoping that it is making a contribution to gas exchange. Is this a valid concern?

[kstagg]

I'm interested in this thread, so I am hitch-hiking along for the ride.

[EricHugo]

Gosh, these are both sort of beyond any sort of factual answer, and my response will be based on mainly anecdote and experience, so with that in mind:

I find the tank responds more favorably to large water changes, but I think its because the change is more obvious whereas with small changes, it may not be as perceptable. I think this depends a lot on the water chemistry of the individual tank, too. A well run tank will not be stressed by a large water chnage the way a poorly run tank with a very long period elapsing since the last change. I have never found anything to suffer, bleach, or otherwise be harmed by even very very large percent water changes, even when done shoddily as I have, at times, been famous for doing. Not that I recommend doing that. However, I have not heard in recent memory of anyone having problems doing the same, except only occasionally and there were some very extenuating circumstances.

On skimming and aeration. I hear all the time about the amount of oxygenation that occurs with skimming, and yet do not recall ever seeing anything to support that positon, though it makes sense in some cases. Again, it will depend on the tank. I think most of the oxygenation happens across the water surface and from large surface areas of water being exposed tumultuously to air. Overflows would count here, too.

I often wonder if the posed "oxygenation of a skimmer" would be greater, the same, or less without the skimmer but just keeping that same pump in place. i.e. is it do to the injection of air nto water or just simply the increased water flow? Anyway, given a stagnant body of water with or without a skimmer, obviously the skimmer will add something. The amount of oxygenation required, however, again depends on the individual tank, so no "recommendations" are possible. Tanks with high nutrients (and higher bacteria), or heavily stocked tanks (corals, inverts, fish) with small water volumes may have very serious oxygen lack at night. Also, tanks with reverse daylight somewhere may be less prone to nightime deprivation. Photosynthesis provides a lot of O2 to the water The best way to know is to but an oxygen test and test the water. I consider this to be a very serious issue for all of us.

If it is a concern, it can always be corrected with more surface turbulence using pumps/powerheads, too. I don't skim and have O2 near saturation all day and all night. But, I have a lot of water flow and a lot of surface area and a lot of empty water space and a lot of tanks connected with a lot of water "falling" from tank to tank.