How often do you change the water? That is usually the source of most problems along with overfeeding. To better help you we need the tank water parameters - Particularly ammonia, nitrite, and Nitrate if your water changes aren't weekly. Also - pearl gouramis can get way too big for your 30 gallon and cory cats are a shoaling species who should be in groups of 6 or more. All this put together can create alot of problems.
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