I had 12columbain blue tetras in a 50gallon for 14month in the last 4months they have started dieing one after another start with no symptoms they are eating ok swimming ok but then just start lose balance and can't swim and are dead with in a day all my water conditions are fine ph6.8 nitrite 0 ammonia 0 nitrate 10I have very soft water kH 3 GH 4 temp 25.5 treated with maracyn2 and done 2 rounds of general cure and aquarium salt has had no effect one Keeps dieing every and 4 to 6weeks i have have treated for bacterial and parasites am out of ideas one of my fish stopped eating to night and is hiding in the plants the odd thing is i have 10xray tetra and 2 bristlenose in same tank and have had no problems with them it's only affecting my Columbian blue tetras