Good answer from Mike there.
besides the 75 not really being big enough for 2 oscars, I need to mention that Oscars do tend to fight. Right now you have two and a third would be like a third wheel of sorts and two oscars would team up against one and you would have some real problems there, and that is in addition to water quality problems down the road too. With any luck, you may have a male and female now and they may live in relative harmony but even at that should you have two males, you're going to have some scrapping in your tank. Definitely don't add a third in my opinion.
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