Partial Water Changes - Effective?

Madratter

New member
Executive Summary:

Conclusion: Partial Water changes are not very effective at reducing the eventual buildup of undesirable elements in our original water.

Corollary:

You want to use the best original water source you can get your hands on because undesirable elements are going to buildup in your tank even with fairly massive water changes each week.

Conclusion: Partial Water changes are effective at setting a maximum concentration of undesirables that are not added in the water itself (food etc.). The faster such an undesirable accumulates, the bigger the water changes needed to get a particular maximum concentration.

Supporting Argument:

I have been thinking over the issue of how much water needs to be changed in a nano-reef (actually it applies to regular reefs too).

In general, no matter how good our source of water, there are still some undesirable elements within it. As a result, there is going to need to be some level of partial water changing to keep undesirable elements low.

Say that you run your water through a RO/DI and at the end of the day you end up with water that still has 3 ppb of some undesirable (perhaps copper). Call it X.

When you originally setup your tank, it is going to have that 3ppb of undesirable X.

However, in practice, X is going to increase from at least two sources.
1) Feeding
2) Top offs of your water

Obviously, if you do nothing to export undesirable X, it is going to increase over time, potentially reaching the point where "Bad Stuff Happens". Perhaps nutrients reach the level where Hair Algae can grow. Or perhaps copper gets to the point it starts being toxic to your inverts.

So lets us take some examples.

Say you have a 20 gallon tank that started with your baseline of 3ppb of X. And suppose you do top offs of 2 gallons a week. You started with 3 ppb. You added 2/20 x 3 ppb of X. So you end up with 3.3 ppb of X after one week. That is actually building up pretty quick. In 10 weeks, you have about doubled undesirable X.

How much water must you change out to get back to 3 ppb. Believe it or not the answer is 100%. This is because your partial water change still has 3 ppb of X in it. Your tank water is at 3.3 ppb. If you leave even 10 percent you get:
90% at 3 ppb
10% at 3.3 ppb

New ppb = 3.03 ppb!

Ouch!

Ok. So I think it is pretty obvious that no one wants to do a 100% water change. That wouldn't be good for the livestock, would put a huge crimp on our wallet, etc.

Ok, so let's say for argument that we do a partial water change each week equal to the amount of top off water we added.

Again, with a 20g aquarium doing topoffs of 2g a week:

Water in tank is now 3.3 ppb.
90% of 3.3 ppb
10% of 3 ppb

New concentration is 3.27 ppb. We barely made a dent over doing nothing where it would have been 3.3 ppb.

If we do 50% water changes:

50% of 3.3 ppb
50% of 3 ppb

We are now at 3.15

Well at least we have slowed down the doubling of undesirable X to around 20 weeks.

Conclusion: Partial Water changes are not very effective at reducing the eventual buildup of undesirable elements in our original water.

Corollary:

You want to use the best original water source you can get your hands on because undesirable elements are going to buildup in your tank even with fairly massive water changes each week.

So what are partial water changes good for?

Answer: They are good for getting rid of undesirables that are imported but not in the original water (i.e. food). An example would be nitrates. But even there, they are not as effective as you might assume.

If your tank produces 1 ppm of nitrate a week and you do 10% water changes:

90% of 1ppm
10% of 0ppm

We are now at .9 ppm of Nitrate. The next week

90% of 1.9 ppm
10% of 0 ppm

We are now at 1.71 ppm

Next week:
90% of 2.71 ppm
10% of 0 ppm

We are now at 2.43 ppm

This will continue until we finally reach 10 ppm of Nitrate. At that point:

90% of 10 ppm
10% of 0 ppm

We are now at 9 ppm. After a week it goes back up to 10 (we added 1 ppm). So we have leveled off at a stable 10 ppm.

If we were doing 20% changes and adding 1 ppm a week, it would level off at 5 ppm.

Conclusion: Partial Water changes are effective at setting a maximum concentration of undesirables that are not added in the water itself (food etc.). The faster such an undesirable accumulates, the bigger the water changes needed to get a particular maximum concentration.

Probably few readers stuck it out to this point, but if you did, my thanks for your time.

If I messed up the math or something, feel free to correct me.
 

rehype

New member
Interesting...I think your study supports why forms of nutrient export and mechanical filtration are very important in enclosed systems. skimmers,chemipure,refugiums and phosphate reactors all are very effective at stabilizing X in each of your scenarios(especially when used conjunctively)

However i will say that partial water changes are very good at replenishing essentials that are needed for certain coral growth and development. I would say they are essential if one isnt dosing or running reactors to replenish lost trace element(magnesium,calcium etc)
 

Madratter

New member
I have rethought this. There is actually an upper limit to how high the concentration of undesirable X will rise when it is added as part of the water. That limit will depend on how fast you are adding it with top offs balanced by how fast you are depleting it with partial water changes.

If you are adding 20% top off water a week at 3ppb of X, you are effectively importing .6ppb of X a week. This is identical to feeding the water .6 ppb a week of X if there was none of X in the water.

If you are doing 10% water changes, you effectively limit your buildup to 6 ppb. At that point A 10% water change reduces the concentration to 5.4ppb. Adding .6ppb the next week you are again at 6ppb. You have reached equilibrium.
 
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