Effect of water changes


Premium Member

I'm having trouble with the math you applied to water changes in Table 4. Take, for example, tin. In each 3-week period I'm adding 833% of NSW values. If I change 20% of the water, I have about 666%. So far we agree. However at the end of two cycles I have (666+833) * 0.8 = 1200. After three I have (1200+833) * 0.8 = 1626. At the end of 10 cycles, I have about 2975. At 40 cycles it has stabilized (833 in, 833 out) at 3332. Compare that with 53,550 in your table.

While I have to agree that 3332% is very high, perhaps even unacceptably high, I can't agree with the conclusion that changing water doesn't make a big difference.
Hi Stuart,

You might be right. Let me check my spread sheet and if necessary I will have the table revised.
Hi Stuart,

You are absolutely correct.

<b><font color="deeppink">THANKS!!!</b></font> for finding these errors. I have corrected the table and text and they should be posted within a day or so.

To a very real extent, however, even with regular water changes the values are still so high as to utterly absurd; they just aren't astronomically utterly absurd. :D

I just got back from doing some research about many of these toxic materials, and have found that our tanks register values of these materials so far about natural values that they even exceed the highest values found in the pollution literature (Marine Pollution Bulletin, etc.).

I'm glad to help.

You realize, of course, that we're all counting on you to find a solution to this problem, right? No pressure. ;)

Now if I just had an infinite supply of NSW in my back yard. Maybe it's time to move to St. Croix, get a nice place on the beach...
Hi Stuart,

mmmm, yes.... St. Croix sounds sooooooo good right now, as it snowed again yesterday.

I think the solution will be better salt formulations, but I think this will also cost a lot more.

Better salt would be nice. Looking at your test results, it appears that the salt is a major contributor to most of the elevated components.

Assuming the salt manufacturers would respond to an appeal from the community, it would help if we knew what to ask for. The fewer changes requested, the more likely we'll get them. As a methodology, I would start with the elements that are shown to be elevated in both the aquarium water and the salt mix. Then I would prune out any that have no known biological effects. I think you end up with a short list.

Once we know what to ask for, we need to give the manufacturers some way to make money by producing a premium product. Maybe you should offer your name to the first product that meets your specifications. :D Or maybe RC needs to have a "stamp of approval" like Good Housekeeping or the ADA.

Looking at the math a little more, there is a simple formula for determining the equilibrium point for an element.

E = (N * (1 - C) / C) + R

E = eqilibrium level (faction of NSW)
N = amount added between changes (fraction of NSW)
C = fraction of water changed
R = concentration (fraction of NSW) in the replacement water

And, BTW, if you assume that a 2X NSW concentration is the maximum allowable for a minor element, you can't add more than 25% of NSW values between 20% changes.
Hi Stuart.

Many thanks!

May I make a suggestion: That you take the tale of your derivation and the derivation itself and build a short article about them and the ramifications of the formula and additions to tanks, etc.

Submit this to Reefkeeping for editorial review and potential publication (for pay...:D).

You have obviously given this a lot of thought, and deserve the credit for it.

Thanks for the vote of confidence! It means a lot to me. However, this time I'm going to pass. Next year I might have more time to think about writing articles. After our house is finished and my 300G is up is running, I might, for instance, write about the principles behind the RCSD I built into the tank (assuming it works as well as I hope).

In any case, this material only makes sense within the context you have created. Please feel free to use it.

BTW, in a few months (maybe not until September) I'll have the 300G up and running... do you have any interest in receiving monthly water & export samples, along with the analysis fees, starting right from the beginning? If you're interested, I'll measure everything that comes in and goes out.