Since you have a sump - you can put some macro algae in there that will consume nitrates and release nitrogen. I am sure that someone else will tell you the exact types as I am not too familiar with them...
In case you use sponges in your filtration water, make sure that you rinse them in aquarium water that you take out of your tank when you do the water changes... This will decrease the amount of waste in your tank and will lower your nitrates
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