I've got a 5g heavily planted tank with some very active shrimp + one snail. Every fish I've placed in there has died. A male betta seemed to have fin rot + died. A female betta died - no visible weirdness. Two male guppies, one right away (couldn't have been the tank) the other a week later (probably the tank?). I've checked all the parameters (ph, alkalinity, nitrate, nitrite, etc) and everything reads fine. The tank has been up for awhile (5 months, now?). The one questionable thing that I do with the tank is use ro filtered water for the water changes... A fishy guy said that the water might be too "pure" can that be?