Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
The water in the tank has a certain level of minerals, measured as GH (general hardness) and KH (carbonate hardness) and TDS (total dissolved solids).
The livestock are adapted to this mineral level. (pH is secondary, not as important as mineral levels)
The animals can handle minor changes in mineral levels.
When water evaporates it leaves behind the minerals. When you top off the tank it is good to use RO because the tank already has enough minerals.
This is a small changing level of minerals: They are gradually more concentrated as the water evaporates, then diluted when you add RO. But you are not topping off the tank by more than 10% of its volume (I hope!) and the changing values of GH and KH might be less than 1 degree. Certainly less than 2 degrees. Even the most sensitive fish can handle that much. (I do not know about shrimp, but I would suspect they are OK with this, too)
A water change is different.
Looking at GH, KH and TDS before and after, if you just used RO to refill, then the mineral levels are much lower because you removed minerals when you siphoned out the old water, then did not replace them when you added pure RO to the tank.
The change in mineral levels is too much for the livestock to adapt to in one go, and they may die. They are especially sensitive to water becoming softer (reducing levels of GH, KH and TDS)
Here is how I would do this:
If the current tank water is too hard for the livestock I would make up a blend that is correct. Proper levels of GH, KH and TDS. (several ways to do this).
Then do 10% water changes daily, or 20% every 2-3 days, using the new blend to refill.
The change in any one day should be smaller than 10% softer, but done every few days the overall trend is for the water to get softer. By the end of about a month the water ought to be right where you want it.
Most fish are just fine if the water is made softer in small increments. Measured by TDS, GH or KH they can handle water that is 10% softer in any one water change. You can do 2 of these water changes per week.
KH is a buffer than controls the pH.
If the KH is high the pH tends to be high and difficult to change.
If the KH is low the ph tends to be low, but is easily changed by other things in the water. If there are other things in the water that make the pH high, then the pH might be high even if the KH is low.
My own tap water is an example.
KH tends to be around 4 degrees (pretty soft), but the pH is in the upper 7s because of the sodium hydroxide added by the water company. Acidic water (low pH) corrodes the pipes, so they make sure the water is alkaline (high pH).