2 months?..would guess too much stock in a uncycled tank...most new comers start off with something like this for a story...bought new tank, pet store employees said to let the tank "cycle" for 24 hours and then add fish..too bad that doesn't work out in the fishes favor very often. Short version is that a tank needs to build up BB (beneficial bacteria) in order to break down the waste given off from the fish, excess fish food ect, if you hurry too much this can cause stress/kill fish as one of the first by products is ammonia. others will pipe in as well and offer some links to help you along.