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20ppm nitrate is very good, but since you have algae problem, you can always aim for 10ppm, or even below 5ppm. Increase your partial water change to a higher percentage will help.
Since you are going to get a few more oto catfish, the bio-load will increase, and it makes sense to increase the water change too.
Do not forget to quarantine your new fish if you have a quarantine tank. It is a great way to discover unwanted diseases or parasites before they can have a chance to infect your current fish.
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