What are your water parameters (ammonia, nitrite and nitrate)? What is the temperature of the water? What type of plants and how many? Your male guppy sounds like he's not feeling too well but it is kind of hard to help you without additional information about his environment.

Yes, water parameters are important to know. Is this aquarium cycled? sorry I can't remember. If not or you had an ammonia or nitrite spike it could be finrot. This often happens from stress from toxic water parameters. Finrot can look like a bite. Does the fish have just "bites" on the fins? Is there any sign of a "bite" on the body? Either way look up finrot on the web if you are not familiar with what it looks like.

What is the saline content of your tank? Guppies need salt. Adding salt also will boost the electrolytes in the water, helping to relieve some of the stress of being transferred to a new tank.

I would imagine once your fancy guppie has had a few chunks of it's fins, the others become aggressive and gang up on him. My question to you....is why would you put beautiful flowing fins fancy guppies in with a male Betta? The male Betta can be with a community tank as long as the other fish do not have flowing bright fins. The Betta male will be come aggressive of his territory. Also female bettas can also be very aggressive with the male betta to the point of death. Before you add any more fish to your tank, you need to check the internet for what is compatible with the fish you desire. Again 1" mature fish for every 1 gallon of water if the rule.
Theresa

can someone explain the guppies and salt issue. thanks.

Fairy toadmother,
I never added salt to my guppy family. I am totally clueless to this but again i am always finding new ideas. Now if you are talking about salt treatment for goldfish and koi, I know exactly what you are talking about.
Theresa

Many guppies are bred in brackish water conditions... thus one of the reasons many people take them home, put them in cycled tanks, they still wind up doing poorly or dying.

I regularly use salt in my guppy tank. I've found they do much better w/ some salt as opposed to totally freshwater conditions.