How long has it been since the last water change? Really the only way to reduce nitrates and other toxins that build up over time is to do regular partial water changes. High NO3 and a lowered pH can be a sign of OTS - old tank syndrome - which occurs after an extended period with no water changes. This isn't the only reason your NO3 may be raised, but no matter what the cause, doing a large water change can shock the fish, it's best to start slow. Change 10% per day for several days, then 20% for a few days until things are back to normal. Once you have NO3 levels where you want them (I don't like to see NO3 over 20ppm myself), start doing weekly water changes of 25 - 50% - as much as required to maintain your NO3 levels at a low level.