You said in the first post that the betta and his friends have been moved to a new tank. Is the new tank cycled - did you move the old tank's filter over, or cycle the new tank's filter before moving the fish?
If everything in the new tank is new, you need to do daily water changes regardless of the betta's tail. Without a test kit you cannot know what the water conditions are, so daily water changes are the way to go.
Cherry barbs are so peaceful I cannot imagine they would ever injure a betta. He may be stressed - you need some plants and a backing on your aquarium to make them all feel more comfortable. Skip the zebra danio's - I had one eat the fins off of a large Gorami and then proceed to kill all his danio tank mates - I moved him to a small aquarium where he is outnumbered by a school of Rasbora's - no killings yet.
I use Hakari pellets - if they are full of fillers, no wonder my larger fish eat them like crazy. My smaller fish (like Cherry Barbs rarely ground feed and aren't strong enough to munch the pellets. I use a specialty color improving food through a company called Cobalt and boy are all my fishes color's improved. The company makes a healthy floating pellet for Betta's But none of it will fix a tail.
Hope the water changes and time and lower stress levels will heal the tail - how long has it been since it happened? If it's been awhile I too would suspect an infection but I would think you would see some signs of it in the tail - can you see close enough to tell what the fin area around the tear looks like? If it looks healthy I would wait and see - if it's starting to look "odd" then try an antibiotic. Remember to remove any charcoal filters you have or it will absorb all the antibiotics and don't give more than 1 antibiotic at a time;