Here's an article on "old tank syndrome" which I believe would be the situation you would find your tank in at this time. As the article states, this doesn't have anything to do with the age of the tank but does have to do with the age of the water in the tank and the parameters they are at. I think this is probably a good article for information. The water change part is the hard part...some say go slow, some say get it fixed right now. Going slow will take forever (try the math)....today I change 30% of the water out, I wait one week and change 30% more... the original changed out water has now had time to become dirty water and my effect from the 30% change is minimal...but as some point out, too drastic of a change can also cause problems.
Old Tank Syndrome
When I was really new with my fishkeeping I once had a tank that had crept up to 80ppm on nitrates (this was when I did not understand how the nitrate test worked and kept getting a 0 reading because I was doing it wrong). The tank was terribly underfiltered and overstocked. To correct the problem, I did a 50% water change on
day one, on
day 3 I did a 30% water change, on
day 6 I did a 50% water change, and on
day 9 I did a 30% water change. It took a while at this rate. I also added more filtration. I was able to get the nitrates down and all the fish survived without ill effects (these were cichlids not small tropicals like tetras or threadfins). This was part of the reason I became such an overfiltration, scheduled maintenance, and scheduled waterchange person with my tanks.
There is a lot of information available on nitrates and nitrate removal on the web. As far as I know there is "no" miracle product that will totally clear nitrates from the tank water and even if there was, water changes serve another purpose. They replenish many minerals and trace elements if you are using tap water, they lessen the chances of fish illness due to less stress in the fish themselves caused by poor water quality, they reduce other illnesses like fungus and parasite problems, and finally think like its you....I wouldn't want to drink or bathe in water that had not been cleaned in a year so why would my fish want to live in water like that. Fresh water is a critical component of healthy aquariums.
One other link that might be of help (I can't find the one to the big article that I have and the one I used to determine water change schedules for my tanks) but this one will work and help some I think to give you an idea of what happens to your water over time and how to effectively change out the water on your tank
Effective Water Change Calculator
Found the link for the article I was referencing...the chart in this link is an excel spreadsheet but it works pretty well and I think it is probably more right on than the above quick calculator. Eye opening stuff really.
The Waste Paradox
Anyway, in the end no criticism here or from me as we all continue to learn about our hobby and how things work. It's a process for all of us.