my botias are wrestling

[melonhead]

ok, so i have 6 botias:
2 - Botia striata (Striped Loach / Zebra Loach / Thin Line Loach)
2 - Botia dario (Queen Loach, Bengal loach)
1 - Botia almorhae (Yo-Yo Loach / Pakistani Loach / marble botia)
1 - Botia Angelicus (Botia kubotai / Polka-dot botia)

we recently merged them into a 55 gallon tank. originally, the yo yo and the polka dot were together, and all the others were together. the polka dot (bigger) used to chase the yoyo around, but it looked like playing, and yoyo would look for it, and never got hurt.

so, now all 6 are together, and the polka dot had taken to chasing one of the zebras. instead of just chasing him around the tank, they chase each other in one inch circles--looks like they're wrestling.

yesterday, i noticed that the zebra's top fin is all frayed! the polka dot must've sucked the skin off.....and they're STILL wrestling, and the zebra doesn't hide or anything...he even instigates!
there are DOZENS of hiding places in this tank. almost too many. they've all explored, so they KNOW there are hiding spots. what is going on? the polkadot used to be the most timid fish. why is he doing this? and it's just the one that he's picking on. he doesn't bother the other zebra. and he doesn't really go looking for him--the zebra comes to him. has anyone experienced this before? i thought these were non-aggressive.
thanks.
LP