If you are having issues with a fish disrupting the balance in the tank, then removing from the tank is certainly an option. You tank should be plenty large enough to accommodate them all. As far as bioload is concerned, if all you have are bettas, Corys and the pleco, you're no where near the tanks capacity. However, the real question is whether your filtration is up to the job. As long as you are getting 0 ammonia and 0 nitrites when you test, you're fine. 0 is 0. As long as you can keep your nitrates under control with maintenance, bioload should not be an issue. If you start seeing ammonia and/or nitrite on a regular basis, the adding to your filtration capacity will fix that. But as I said, 0 is 0.
Many behavioral issues with fish are due to external factors. Diet is one of those factors. Plecos are omnivores, so if the are not getting enough protein in their diet, that can be manifested in various ways, such as aggression and later slime coat eating. If they are just getting algae wafers, then that might be part of the problem. Corys are primarily carnivores, eating worms and inverts and such living on the bottom. While they may eat algae wafers, algae is not a big part of their natural diet.
Also, one of the most effective ways of dealing with good aggression is to feed small foods. It's difficult to hog all the food when it's everywhere. I just said all this almost word for word in another thread this morning.
Before you go and give your fish back, I suggest changing the fishs diets. Try feeding a high quality pellet, like NLS, as the bulk of the fishs diets. They can/will all eat it and love it. I think you will be surprised at the impact such a small change will make, though its really a monumental change. Diet is very important; feeding the fish the highest quality food goes a long way.
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