What kind of rock do you have. Is it porous reef rock? After 2 years it should be populated with anaerobic bacteria. You could try dosing CAREFULLY!!! Dosing can cause bacterial blooms, stress your fish, totally bottom out your nitrates and reduce the oxygen levels in your tank. I have killed fish and inverts by having a dosing pump miscalibrated. However it does work. I use simple sugar. Some use vodka and their are a lot of dosing additives out there. Just be careful if you chose this route. Small doses/measure accurately/ small dose and so on. Just remember the dose to get you from 100ppm to an acceptable level is not going to be the same as a maintenance dose. Keep in mind you have to maintain both nitrates at a balanced level in your tank or bad things happen.