Please list the value from each test you did with the test kit. Ocaationally we seee people misinterpreted the raadings. How much water do you change out and how long do your go before another change.? Do you dechlorinate the water before you add it to the tank? Were the fish behaving normally or looked abnormal just before hey died?Bought test kit - all the levels are right.
Change the water regularly
well bought it from the place we bought the fish.Was the gravel you used designed for fish tanks? Some non-aquarium coloured gravel can leech toxic dyes into the water.
If your GH had increased, something is increasing it. Could the substrate be dyed coral or limestone?