Why are my gourami and plecostomus other biting eachother?

they've been together for years. their tank is big enough for both of them and their sizes. and, i pass them plenty of food.

the gourami's fins are getting frayed. why are they fighting so much?
You don't say what type of tank or what type of pleco or what type of gourami you have. If they are both mannish, and have both decided that they want the same article of the tank, they could be squabbling about that.

Here is a web site on gourami's
http://freshaquarium.about.com/cs/anaban...

and one on plecos
http://www.fishlore.com/Profiles-Pleco.h...
Well first off all that tank ISN'T big plenty for both of them, we've already covered that.

Aggression issues are a sign this is a problem, neither Gouramis nor Plecos are usually overly aggressive towards other species, so yes, your tank is not happy.

And whether they've been there for "years" I'll bet there's some stunting also, you still haven't said what species you have precisely.
Answers:    Some plecos develop the tendency of trying to feed stale other fishes slime coat. They learn this behavior especially if at one time they be in a tank with dying fishes(they swot up other fish can be a nice meal)..
Your gourami is probably trying to defend himself from the pleco approaching him, plecos do grow quite big!! I had at one time literally cull two plecos and bushynose plecos as economically. For the most part they tend to behave well but plecos do inevitability lots of food...
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