Could be but the filter being new as you say it is should have had no ill effects on your tank.All it would do at this point is keep the tank clear and clean since there is no bacteria colony inside of it yet....You described your AT breathing heavy which might have been a sign of parasites that it could already have been infested with, and finally succombed to.shua71;4875900; said:Hm so the main cause of this is that I'm over stocked? Not that I put in a new filter?0