What test kit are you using? I did not think there was a color indicator for .75ppm ammonia on the API master kit, but if you are showing levels of ammonia and there is no conversion to nitrate (i.e. test readings are not showing nitrate or nitrite) then it seems like your tank is not cycled and your fish are suffering from ammonia poisoning. How did you cycle your tank or how long has it been established? I seem to remember reading a thread about you toying between the options of "fish-less" and "fish-in cycling". If you chose the latter then you better be doing daily 30% (bare minimum) water changes to keep ammonia levels down and fish alive.
I put in three cories with substrate from an already cycled tank. I guessed .75 because it is between .5 and 1 ppm. I am using the API master test kit. I would change water but there is chloramine in my tap water which is partially ammonium so when I change water it just adds ammonia or keeps the level the same. I acclimated them by floating the bag for 15-20 minutes, which I now realize is the wrong way to do it. I guess I could return them and wait. I will see if I can do that.