problem with juvenile platys

[crs1945]

I had 3 juvenile platys in a 5 gal. tank. One became very lethargic and died within a day. Now another one has become lethargic to the point where he hides under a piece of driftwood and seldom comes out. He's been that way for several days. Other than slightly faded color, he shows no sign of disease that I can see. On the other hand, the 3rd platy is active to the point of being hyperactive. He's acting quite nuts! Water temp is 76. ph-8.4
GH- 8 NH3-0 NO2-0 NO3-5.0. I do weekly 25% water changes. I don't know what to do for the little guys. Any ideas?