I have about 10 guppies, 2 scissortail rasboras, 1 galaxy rasbora, 1 corydora, 1 platy and 1 pleco in a 55 gallon tank. It seems everyday I find another guppy dead and it seems their tails have been picked. Who would be the culprit? The water parameters are where they should be. Ideas please? Thanks so much!