Why are my fish eating each others tail fins?

They seem to pick on my Black Moors the most. 2 guppies killed 1 already. I removed them but now my small goldfish are doing it.
Answers:
Yea Goldfish Are Said To Eat Live Foods, Small Animals And Other Fish, Adding Worms, Brine Shrimp Or Other Treats Should Take Care Of Your Problem. You'll Need To Feed Them Twice A Day. Just Enough Food That Is Completely Eaten In Under 2 Minutes With NO Leftovers, Anything Left Over Can Lead To deadly water conditions.
maybe they are hungry
The tank is too small for the number of fish. My fish all did this until there was one lone survivor, who did very well for a few years, till fin rot set in.
Im not completely shur, but i would get friendlier fish as tankmates. Guppys are fast so, they might pick on otherfish. I had a black moore when i was really young.i cant remember much about them, but i think mine was always a slow swimmer. Try putting it with a male or female betta. Mine is with gouriami, glass catfish, and swoardtails. Bettas arnt too fast at swimming either. Bettas really dont fight too often, occasionally theirs a fish they dont like, but it's a myth. !!WAIT!! They could be attacking because the tank is too small. Each fish needs a gallon of water per inch of it's body. Or, if you didnt see them attack, but fins are frayed and continue to be, it could be finrot. Finrot is from poor water quality. Go to your petstore for advice on medicine(to keep fungus from attcking), and to see compatible fish!! good luck.man.i write a lot!
idk why..do you feed them enough?

maybe they learned it from the other fishies..but arent fish kinda dumb..idk
Cannibal fish; rare but not unheard of
Here is part of an answer I used in another question, different problem but answer is the same.

Begin proper feeding of your fish. This may sound funny but there is a right way and a wrong way. This is by far the most common question I get "How do I feed my fish?". The answer is simple, feed fish A LITTLE AT A TIME (they should easily eat all the food you give them) untill ALL fish are done eating. If you do it right most fish will feed for about 5 minutes before they refuse food. You will notice a few fish feed like crazy, not even letting others feed. Those are your dominate fish. If you have ratty tails on any of your fish it was most likely done by one of these dominate fish. The dominate fish always feed first, then they will stop feeding and the others will get their chance. That is why it takes around 5 minutes. A little at a time means a small pinch by the way. Give a pinch, wait for the fish to eat it all or most of it (the idea is to have no waste) then give another pinch and so forth. If anyone ever has high fish loses this is the most likely cause, feeding too much, too fast or not enough. Flake food is great for learning to feed, once you have the hang of it you might switch to a pellet food they have more nutrition per bite. Frozen foods loose most of their nutrition when ice crystals form in the cells and rupture them.

Most likely your fish are hungry but it isn't uncommon for black moors to get picked on in general, not sure why, but it has been something I have noticed over the years.
1. You might need to feed them more?
2. there could be to many of them.
I think they are not fed enough. Just give something more but not too much to cloud the water. Have a bigger tank or some stones or objects for hiding will help cos they don't get to see one another too often as in a small tank.

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