Water parameters are ideal (ammonia 0ppm nitrites 0ppm Ph 7.0 nitrates 10ppm), neon tetras and celestial pearl danios seem perfectly healthy but my guppies one by one have died off... Only thing I can think of is that I don't have an airstone but the spray bar on the Fluval Flora makes enough water surface current. The guppies start looking very sick, lose their color and are lethargic before they just die.
sounds like a case of GH and KH not being high enough for them. pH however is good, but make sure it doesnt go acidic at night in case you inject co2. guppies are extremely hardy, but must not be kept in acidic softwater conditions. also maybe you need to acclimate them better to your tank.
As always make sure to buy HEALTHY stock from the start. bad stock will die no matter what you do because the source doesnt take very good care of them and they get very weak.