Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
To make the change from tap to RO that has been remineralized, I would follow either of these paths. The new water could be a blend of RO + tap water, or pure RO with minerals added.
1) Make the new water so the GH, KH, TDS, pH match the existing water. (not more than 10% softer) Then do as big or as small water changes as you want.
2) Make the new water different and make smaller water changes so the net effect is that the GH, KH, or TDS is not more than 10% lower than the fish were used to. You can do 2 of this type of water change weekly until the fish are in the new water. This may take time. That is OK. The fish metabolism takes up to a month to reset to a significantly softer water.
First option: If the current GH is 10 degrees, then make the new water GH not lower than 9 degrees for one water change, then 8 degrees, then 7... and do 2 of these water changes per week. Water changes can be any size.
Second option: If the current GH is 10 degrees, but the new water (whatever blend you are using) has a GH of 5 degrees then you could do a water change up to 20%. Net result is tank water at 9 degrees. If the tank needs a bigger water change than this for some reason, use method 1. Keep doing the math each water change so you know how big a water change is safe.
Either way, it will take several weeks to a month to make a big overall change in water parameters.
To confirm that the water change is OK:
Do a water change with water that you think it right.
Give it a few minutes to an hour to circulate, and watch the fish.
Test the GH, KH, TDS and pH. If these numbers are in the right range, and the fish are behaving OK, then that is a good water change. If the tests show the values are dropping too fast, (your new water was too soft) then add a little Epsom salt to correct the GH, or baking soda to correct the KH. (either of these will also correct the TDS).
Note that I am not suggesting using only Epsom salt to remineralize the RO. This is just a quick measure to correct the GH if there is a problem. To remineralize the RO I recommend a GH booster that has a balanced level of calcium and magnesium. Baking soda for KH, or potassium bicarbonate for KH.