Simultaneous Addition and Removal

[Latro]

Do any of you do WCs by simultaneously adding and removing water? I was noticing one of my fish, the smallest one, was swimming badly (away from equilibrium significantly), and then when doing a largeish WC (in which I noticed higher than usual amounts of food and poop; I may need to go BB, heh) I noticed some of my other fish were hiding and darkening, more than usual.

So I set up another hose to add water at the same time; they seemed to appreciate the clean water being added even as water was removed, and two of them even went under the hose. Do you find that this works well? Do you encounter any problems with it?

Incidentally, I did a little math, and purely from the perspective of concentration, you would have to move the same volume of water as about a 70% WC to get a net change in concentration (of something in the tank and not in the input water) equivalent to a 50% WC. (Assuming the flow rates of your two hoses are exactly equal; the math becomes messier, albeit still doable, if they are different). In general, an x% WC would require 100*(ln(100)-ln(100-x))% of your volume of water to be moved in this way; for example, a 75% change would require moving about 140% of your volume of water (you can see that this becomes impractical as x gets especially large, and in fact becomes infinite as x goes to 100). I suppose someone might be interested in this...

(By the way, in units of fractions, where x=1 is the equivalent of a 100% WC, you just get -ln(1-x) at the end, which is a much simpler expression.)

[mungkee]

I usually fill up my 35 gallon brute trash can with water that is near tank temp, dose it with prime and then pump it in. My discus don't to seem to mind at all when I do that. I've tried filling my tank up directly from the hose and they'll usually start shedding their slime coat. What temperature is the incoming water and have you tried filling at a slower rate?

[raidendex]

I would say the main issue with that would be the need to run a lot more water through to achieve similar results to say just a normal 70%. Doubt any of those simple equations would work well for figuring out exact numbers since the process itself is fairly unpredictable. At first you would pump mostly the old dirty water, but as time would go on more and more of newly pumped in water would be also extracted out, depending how close in and out pipes are at time you may even extract mostly clean water.

Adding a dye to water would be a decent way to figure out how much you need on average (fungus meds would be easiest source for that I guess). Add color - do normal X% water change, note the color difference. Then add same amount of color and try simultaneous water change and see how long it will take to reach similar color.

[Latro]

I would say the main issue with that would be the need to run a lot more water through to achieve similar results to say just a normal 70%. Doubt any of those simple equations would work well for figuring out exact numbers since the process itself is fairly unpredictable. At first you would pump mostly the old dirty water, but as time would go on more and more of newly pumped in water would be also extracted out, depending how close in and out pipes are at time you may even extract mostly clean water.

Adding a dye to water would be a decent way to figure out how much you need on average (fungus meds would be easiest source for that I guess). Add color - do normal X% water change, note the color difference. Then add same amount of color and try simultaneous water change and see how long it will take to reach similar color.
The equations take all of that into account with the one exception being that they assume the tank water is homogeneous, that is that the concentration of everything is the same at any point in the tank. Dealing with inhomogeneity gets into fluid mechanics, which is a task I'm not equipped to deal with. Homogeneity is generally a half-decent assumption for things in solution, though; it is mainly inaccurate right around sources of whatever substance you're talking about, such as food or poop, so if you remove all of that in the early stages of the WC these should provide pretty good estimates.

[Skip]

MCISHAQUE does.. i have watched it

[Rod]

I do it that way on fry tanks. Drain to about 1/4 full then start the pump to add fresh water. I keep syphoning until the water is perfect. Doesn't worry the fish at all.