Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
If you are running the tank with straight tap water, GH 16 degrees, then the fish are used to that. The plants and fish will not use so much Ca and Mg through the week that the GH would go down very much. Maybe a degree. Maybe not even that much.
When you do water changes, then, it is not a significant difference.
If you are blending your tap + RO, and always make the same recipe, then the same idea holds: The plants and fish are using some of the minerals, but not much, so water changes should not be a problem.
If you are topping off with hard water or dosing GH booster, THEN the GH will be rising through the week, and a water change at the end of the week might mean too great a drop in GH, but probably not. However, over several months the tank water may have a tendency to gradually get harder and harder. This is when the volume and water chemistry of the water changes becomes important. It is better to top off with RO, and not dose GH booster in this case.
Here is how to tell:
Test GH at the beginning of the week. (Right after a water change).
Test GH at the end of the week. (Right before a water change).
If there is only a degree of difference, it might mean nothing but user error in testing. Or it might mean something.
Keep on testing, and chart the results. After a month or two...
If the GH has been rising, then there is something contributing to the Ca and Mg, and you probably do not want that.
If the GH has been sinking, then the plants and fish are using more of the Ca and Mg, and you might want to supplement. (easy: top off with tap water instead of RO)
You can follow whatever method you want to dose the dry ingredients.
Dose dry, right out of the package. I toss this into the flow from the filter or power head if I dose dry.
Make a blend in a bottle and dose dissolved ferts. This is faster, so I do this, making a big batch every so often.
Anything in between.