Black Molly issues

Question
    Hello Matt,
We have a ten gallon tank freshwater. No live plants yet plenty of rock and plastic decor. Good healthy algae growing on the plants. Not to thick. Five fish: One dominate good looking male black molly, female black spotted molly, male beta, Cory cat, and eight inch sucker catfish. Our problem is the black male molly biting on the Cory after feeding. The Cory developed a hole and white infection. We are medicating the tank with out the filter in four day increments. The Cory is healing after a week of medicating. Problem is the black molly still likes to pick on the Cory. The female molly was a new addition when the problem started. Local pet store recommended another molly to keep the male busy. So far no luck. Tank temp is average of 76F, PH never higher than 7.2, if PH reaches 6.5 we change 50% water. We are aware of the suckerfish(Plecto?) does release a lot of ammonia/urine and we change half water once a month with a new carbon filter. We feed the tank lightly every morning. We love our black male molly. He seems to be the alpha of the tank. He is strong and thick for his size. Also has a great full dorsal fin. Have not seen one like him at the stores. Very bad ass looking. HOW do we stop him form picking on the Cory cat. He only picks on the Cory. Please help us dude.
Look forward to hear your advice.
Best regards, G-Florida

Answer
Hi George,
Unfortunately there is no way to keep him from doing that besides upgrading the tank for more space.  He is most likely territorial.  Fish are just being fish and be ended up being an aggressive type, especially towards that cory.  You will need to separate one of the two.  Have you thought about separating the cory?  And yes that pleco does have a large bioload.  It is also a big fish for a 10g tank.  A small tank will stunt his growth, cause stress, and health problems.  A bigger tank will help prevent it.
-Matt-