On my nano tanks I do two 25% water changes a week. That's with super good filters, live plants and loads of bacteria. With yours at max I'd do one 50% once a week. Otherwise all the bad stuff just accumulates in the tank and will eventually make levels unstable and kill most of your stock. Trust me on this. I made that mistake once on a $200 pair of Arowana. Not such a good thing pulling dead plant matter and fish from that tank. So I wish you good luck as we have probably given you sound advice.