My 80 gallon tank with about 75 gallons of water has high nitrate. With my API test kit, it's somewhere between 40.0 and 160.00. My opinion is that 'old tank syndrome' is just another name for high nitrate.
I know from past experience that only high % water changes actually reduce nitrate. Low % changes accomplish doodle squat. Unfortunately, because I have to adjust my source water to increase pH, carbonate and calcium, high % water changes are not a viable option for me. So I've tried a few things that so far have not worked.
Now it's on to API Nitra Zorb. I just added two pouches in a Topfin Power Filter 75, which I've had for a few years and have used for various filtration tasks. I purchased 4 pouches at CAD14.45 each, which is about US$12. The instructions are to recharge the individual pouch every 5 days. I bought two sets so as not to interrupt the filtration process while I recharge the used pouches each week. The pouches are supposed to last 2 months before discarding.
I will track (any) progress and post the results here.
Apr03/16 Nitrate: 40.0+