algae eating fish - can they attack???

I have a albino bristlenose pleco...

every so often I drop an algea chip in the tank for him and my algea eaters along with my cats...

If and when my bristlenose finds the algea chip he doesnt let any other fish near it at all... he even pushes away my bull shark... the bull shark is about 3" and the bristlenose is about 2". Other fish try getting a nip of the algea chip but if the bristlenose is there he pushes them all away...
 
ive actually seen this, my friend who didnt know how to keep fish or how many fish to put in a tank and would do the opposite of what i told him had a 8" common pleco in a 10g, he eats only tubifex worms. my friend put in 2 oranda goldfish, about 3-4" probably and with in like 5 minutes the pleco was sucking on the side of one of themwhile the oranda was swimming. it just sat there sucking and sucking i immediately took the other 1 out and wee took it back. about 2 months later he gave the pleco to LFS.

it was crazy to watch a pleco jsut kill an oranda like that!
 
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