For my 40 breeder, I do 15% water changes each week. I mix the water (about 5 gallons) and let it sit for a week. Currently, I use the following concoction:
-- 5 gallons tap water (KH 0-1, GH 2-3)
-- 5 mL API Tap Water Conditioner
-- 1 1/2 tsp Instant Ocean Sea Buffer (sodium carbonate, sodium bicarbonate, sodium(?) borate)
-- 1/2 tsp calcium carbonate (powdered)
-- 1 tsp epsom salt (magnisium sulfate)
-- 1/2 tsp calcium chloride
-- 1/2 tsp gypsum (calcium sulfate)
-- 3/4 tsp Instant Ocean Sea Salt
This gives me a KH of around 3-4 and a GH of around 11 in the tank. I also have a Ca level of about 60 ppm. Phosphates are around 4 pmm (probably from fish food -- I feed a lot of food to a lot of fat fish).
I'm going to start adding some potassium nitrate because my nitrate is currently 0 ppm.
After the water change, I dose with 20 mL of LeafZone and 5 mL Flourish (one per week). I also add 5 mL of Flourish Excel daily and recently added a DIY CO2 reactor connected to the Nutrafin CO2 generator.
I'm thinking of increasing KH a bit because, although I get KH up to 3-4 after the water change, it is back down to 0-1 before the next weekly change.