Welcome to R2R! As noted above, it depends on how high your nitrates are and what setup you have -- tank size, filtration, water supply, tank inhabitants, etc. Generally, I lean towards multiple water changes of around 15-25% to avoid making too rapid of a change in water parameters, but I believe water changes of 40% are also decently safe depending on your tank inhabitants. If you aren't using RODI water, it might be a good idea to test your source water, as non-RODI water often has nitrates present that will inhibit your ability to remove them from your tank. It's also a good idea to determine where the nitrates are coming from. Are you using non-RODI water, are you overfeeding, is there something dead/dying in the tank, do you have too much bioload for the filtration to handle, etc. When I inherited my tank, the nitrate levels were toxic, with the rocks having absorbed significant amounts that continued to leach out into the water for months. I found eventual success by slow and steady water changes, and dosing carbon (in my case, very small amounts of vodka).
