About two years ago, I brought home a pair of Gold Gouramis for a community tank I was keeping at the time. One of the Gouramis was more dominant and bullied the other, keeping it from getting access to food. Naturally, the more aggressive fish ate better and became larger. The less aggressive Gourami became smaller and smaller until it was one day killed as a result of the bullying.
I continued to keep the aggressive fish, but she as slowly dwindled in size over the past year. I try not to over feed, but I doubt that I could be underfeeding. What could be the cause of a shrinking fish?