I have a 10 gallon low light planted tank with three guppies, a blue paradise, albino cory, and farlowella catfish. Lately, my one guppy has been pretty agressive towards the other two, seemingly bothering them all of the time. He has also had a dark brown/deep red spot on his tail now just past the base of it for a few weeks. I figure it may be a cyst/tumor (he is a tequila sunrise guppy so he is yelow, orange, and red). Anyway, he started acting more aggressive lately and constantly chasing the others and harassing them. He did also start exhibiting some mild flashing behavior. One of the other two guppies would also flash from time to time. I did put a Tetra Parasite Guard tablet in the tank a couple days ago to be safe. The other two guppies treat each other well I might add, it is just with this one that they show aggression. They are all male and I know the male to female ratio rule but I don't want millions of fry in my 10gallon at college. Any suggestions? I switched the tank around a few days ago b/c I removed my gravel and added sand so they should have new "territory". I don't know why they would be flashing though - could it be stress? Water parameters checked out okay. Should I continue to dose tank with the parasite tablets or will that just make the flashing worse if it isn't a parasite? There are no whites spots or anything really visible so it's not Ich.