Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Equilibrium contains potassium, calcium, magnesium (these two raise the GH), iron and manganese (all of these are plant fertilizers).
Equilibrium contains no carbonates, and does not raise KH.
Breeding soft water fish it is very important to have a low GH. Find out why the GH in your tank is rising so much and take steps to stop and reverse it.
I actually go by TDS (total dissolved solids) as well as GH and KH when I prepare water for a water change. If the new water, blended with the original water in the tank, will create small enough changes in TDS, GH and KH, then that is fine. When I need to do a large water change I will put more effort into making the new water match the tank. I do not worry about the pH. This is of much less importance than the other measurements to determine if the fish will be OK with the water change. For small water changes the new water does not have to be so similar to the water currently in the tank.
Gotta do some math to figure it out, but TDS, GH and KH are all straight line values.
Figure out the % of new water x each value
Add the % of old water x each value
then compare that to the original value.
If the proposed water change will keep the difference in TDS less than 10% of the original value dropping, or 15% rising the fish will highly likely be fine.
If the proposed water change will keep the difference in GH less than 1 degree of the original value dropping, or 2 degrees rising the fish will highly likely be fine.
If the proposed water change will keep the difference in KH less than 1 degree of the original value dropping, or 2 degrees rising the fish will highly likely be fine.
Takes a while to write it out, but here is an example:
Old tank water TDS = 400 ppm
New water (being prepped in garbage can) TDS = 300 ppm.
Proposed water change: 50%
Question: Is this safe?
Answer: the tank will end up with 50% @ 400 ppm = 200 ppm and 50% @ 300 ppm = 150 ppm.
Add 200 + 150 = 350.
Difference between original TDS and new: 400 - 350 = 50 ppm.
Find: 10% of old TDS = 40 ppm.
Question: is proposed water change going to create a difference in TDS of less than 40 ppm? No. 50 is >40. This water change is more than 10% less TDS than the fish are used to. This is not a safe water change.
I actually have the formulas on an Excel sheet so I can figure it out faster.