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Oscars are very agressive fish and will eat small fish. Rule of the thumb is that you put nothing smaller or less agressive than your oscar in it's aquarium. For one thing smaller fish are considered food. Second unless you have alot larger fish like the pacu the oscars may pick on your fish. One fish I have growing with my oscars is a pleco. Plecos tend to be too big for the oscars mouth and seem friendly to the oscars as well as clean up their enviorment. So the oscars do not seem to mind the company of a pleco,however plecos too get to about 2'.
Oscars also are from tropical regions. Which means they like warm water (75-80 degrees) they live well in these type of temperatures. The way you maintain these temperatures is to get a reliable heater for the size of aquarium you have. Keep the oscars enviorment warm by puting in a reliable heater and that will be one less problem you will have with your oscars.