I can't stress how important the water changes are, especially for Oscar's. They are prone to getting HLLE from poor water quality and hard water.
I did 75% water changes every three days.
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I also ended up rehoming her due to my very hard water to a fellow MFKer.
Get yourself some sort of liquid test kit. Most of us use the API master test kit.
With this you can test your water. Learn if you have soft or hard water. You can test for nitrates so you know when you need to do water changes. There is no set schedule for all tanks. Size and number of fish vs water volume vs filtration will determine your water change schedule. You want to keep nitrates below 20, preferably around 10.
Remember each time you change 50% or more of the water you remove 50% of the nitrates.