I have about a 20 gallon tank that contains 3 minnows, an angle goldfish, and I did have a pleco but it died all of the sudden. When I got is I got it and one other for 2 different tanks, and the other one is doing great. There is algae present in the tank and blood worms and goldfish pellets are the food for the tank. Any idea why this one died but the other is doing just fine?