1. Return the bala shark. They are schooling fish and grow to a foot long and so it is a both a poor choice for your tank and the wrong way to keep them. That takes care of the bala problem.
2 Stick to just one type of territorial fish in your tank to avoid hostilities. Angel fish and gouramis are both territorial, so keep one or the other. Choose the gouramis since Angels ultimately outgrow a 20 gallon tank.
3. Make sure you have the tank decorated suitably. This means including lots of cover, especially in the way of plants. Cover gives the fish places to hide, and blocks lines of sight across the tank, which eases both aggression and stress and promotes activity. Bottom dwellers like catfish should have secure caves to hide in.
4. Try not to keep other single fish with your gouramis or they are more likely to target them. Instead choose a suitable type of fish that you can keep in a school, like some tetras or raboras, and get several of them.
5. Gouramis are territorial, but tend to do fine in community tanks as long as you take precautions like those I mentioned above. That said though, sometimes we end up with a fish with an overly aggressive personality and nothing seems to work. When this happens the choice is to remove the offending fish altogether, or face the facts and keep them without tankmates.  | 
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