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about 20-30% weekly would be fine, once you are totally sure the tank is fully cycled. You can change up to 50% safely but with something that big I would go closer to half that because it's a lot to change and you shouldn't have problems keeping the good bacteria established.

17 fish in a 20g seems like a lot to me but someone else will have to comment on that as I haven't set up such stuffed tanks before.
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