I don't think water change is an issue at all. I am already doing 2 50% WC per week. I am not even keeping discus. The issue is that young Discus is prone to illness. Then, you need to treat it with medication.
two 50% on a 90 gallon should be okay. you won't get massive growth.. or maybe even grow them out to their full potential. they'll live healthy but won't be a good fish to brag about ^_^
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