I know it's silly but one of my oscars drove me nuts. I had two small oscars in a 10g while my 55g was cycling. After a few days, the red oscar became really territorial and constantly nudged and even bit my tiger oscar. Initially he only did that under his rock but then he must have thought he owned the frigging tank. I thought that sort of behavior was harmless until the tiger started taking some physical damage. My girlfriend would tap on the glass to break them up but after 2 seconds, the red oscar would be well on his way to abuse my tiger. Also, my tiger is a pushover too!! Finally, yesterday I had no choice but to transfer them both to the 55g earlier than planned. Now they have more room and haven't fought yet. However, if that happens against in the future, can I do anything to stop or discourage that? More toys, different diets maybe (right now their main meals are flake food and pellets).