Mineral Replacement - Adding Water For Evaporation

I have been learning that keeping nitrites and nitrates low is not the only reason to do a water change. Mine have been at zero for about two weeks so I haven't done any.
I have been adding water because of evaporation loss. Today I added a gallon to my 10 gallon tank. So that would be a 10% water change. If I do this amount each week, that would be 20% in two weeks, is that enough to replace the minerals and do all the other things a water change does?

 

I would still do water changes as well as topping up.

By just topping up there could be other things you can't see building up over time like phosphate, heavy metals n that.

 

You won’t be replacing minerals by topping off after evaporation but will be adding to them as the minerals won’t have gone anywhere - they don’t evaporate with the water. Unless you are topping off with RO water of course.

 

Minerals don’t evaporate. For this reason some people top off with RO between water changes.

You may be able to get away with going longer periods between water changes, but I agree with above poster that you should still do them, aside from minerals, other things can tend to build up, and water changes help create/keep a balanced tank.

 

Good info, thatnks.

 

Buckle up, I'll take you on this ride in a direction I don't think many even bother with, nor have thought about comprehensively.

Minerals do get depleted by plant growth and bacterial growth as well as other naturally occurring process. Minerals do not evaporate at a significant rate. If plants are trimmed and removed from the tank, the weight of the plants waste will correlate to minerals, nitrogen byproducts and water. These things will need to be replenished in the tank.

The idea of topping up with mineral rich water as evaporation occurs in a heavily planted tank it's something I have done a good amount of experimenting with. I know one of my tanks uses about 4 dkh and 3 dgh every 2 weeks, another tank uses closer to 1 dgh and 1 dKh every two weeks. The minerals that make up kH and GH (let's call them buffers) are harder to test for individually but I have about 15 different tests and have observed that the rate of each mineral is not a consistent ratio between any of my water systems. Due to this I will keep the rest of this basic and talk specifically about the easy tests, dKh and GH, which can sort of be correlated to a tds, but I will not be discussing tds.

Easy example: If a water supply contains 5dgh and 5dkh, but a tank is using 3 degrees each every 2 weeks, and only evaporating 5%, by topping up with 5 degree source water you would only n be bringing the kH/GH back up to 2.25, the tank will ph crash in 10 days.

Moderate example: source water 5dgh/5dkh. Tank uses 1dgh and 2dkh per 2 weeks and evaporated 10%. Topping with source water will bring tank to 4.5dgh and 3.5 dKh. The tank will survive another ~3 weeks before running out of kH and having a pH crash. At this point the water will be about 3dgh/0dkh. Even a 50% water change will only bring you back up to 4dgh/2.5dkh.

Hard example: we remember that dgh is made up primarily of calcium and magnesium. Source water 4dgh/4dkh, and wee will guess 1:1 magnesium to calcium. Tank uses 1dgh/kH weekly, and evaporates at a crazy fast rate of 25% week. In this example we will see that it is possible to lose minimal but still significant hardness. At the first top up we are at 3.5 dgh/dKh. What we don't know is that our plants used calcium: magnesium at 2:1. Our original 2degrees as calcium and 2 degrees as magnesium had become 1.25degree calcium(usually expressed in ppm but keep it simple), 1.75 degree magnesium, and after top up we now have an imbalance by .5degree. next top up we have a lower hardness again and an imbalance by 1 full degree. If this continues your plants will lose access to calcium before a testable result of deficiency unless you test for calcium individually, hard to do in fresh water tbh, but possible.

The above evaporating rates are crazy BTW. I think I lose about 3 gallons a week across 150 gallons in one rack. This would be 1-2% and obviously the rate of replenishment that way would be insignificant.

Now, you may ask yourself, what if we make stock solutions of various minerals, change our water at known percentages based upon each tanks use of nitrogen byproducts and minerals as tested individually, and add the correct amount of stock solution to hit the target as close as possible, once a year or so we will flush the tanks to reset any slippage as we learn to run our tanks "depleted". Well, this is hard but it works for me. It doesn't work for most people. It's allot easier to change more water than you need to every week, and worry less, but be aware slippage is happening to every one not on a drip system with a rate in excess of 100% weekly.

 

John58ford​

Thanks for all that info. I will have to really check into all this to get my head around it.

Hopefully I didn't miss this in a brief scan of what you posted but does adding plant food help take care of the mineral loss?

 

John58ford​

Thanks for all that info. I will have to really check into all this to get my head around it.

Hopefully I didn't miss this in a brief scan of what you posted but does adding plant food help take care of the mineral loss?

Not all plant foods are the same, some have micro and macro nutrients and would help with mineral loss, some are only NPK and have no minerals. You really need to research it of you're going to approach it this way, or you can go the way most do and do a little more water than required and over dose fertilizers to make sure you're set and light limited vs nutrient limited. Could go on for days about the complexity of this subject and allot of different methods have merit.

 

John58ford​

Thanks again. I think I will be doing water changes while I work on this issue.