I am dealing with trying to lower the nitrates right now. I had surgery that caused me to have to "just" get by with water changes. Now that I have safely healed and the tank looked a little neglected, we (hubby & I), did a 20% on Friday, tested the nitrates again today and found that they were at 20 ppm, so "we" did another 20%. I will retest tomorrow and if they are still high we will do a 10% tomorrow and retest the next day. You can safely continue on with this until they are where you want them, but allow the water to mix together around 24 hours before retesting and doing another change. It would be senseless to do a water change if they come back to zero or as close to as possible. The high nitrates will have an effect on the fish eventually, if they get high enough. I try to normally keep them 10 ppm or under.