Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
I would set the GH to suit the fish. Some Cories are soft water fish, prefering a GH that is fairly low, say under about 5 degrees. The hardiest Cories do not seem to care what the GH is. Hard water is OK with them.
Then monitor the KH and set it about equal to the GH. This is because in nature most of the GH and KH in the water is from dissolved calcium and magnesium carbonates. So the GH and KH in natural water is often more or less equal.
With Safe-T-Sorb you will have to keep a close eye on the KH and probably add something to offset the removal of the carbonates.
I used baking soda to do this.
1 teaspoon of baking soda added to 30 gallons of water raised the KH by 2 german degrees of hardness. When the pH was off the chart low (under 6.0) the pH would come up to 6.2.
I would set the GH and KH right when I did a water change, then about once a week, a few days after the water change, I would add a little baking soda. Since I was not adding much (KH change of only 2 degrees) the fish were just fine with this.
Another way to do this is to add coral sand, oyster shell grit or limestone sand to the filter. I use a nylon stocking. These materials will raise the GH and KH. They do this slowly. Not a substitute for making the water right when you do a water change, but might be enough to keep it stable through the week.