A few weeks ago I also had a spell with high Nitrates in my tanks. I figured out why, half was me, half was the water. I have well water, and I am surrounded by crop land. In the last 6 weeks the farmers are planting and fertilizing. My water had 40ppm Nitrates. I went to Walmart and bought about 100 gallons of Reverse Osmosis water (several trips) It didn't cost much because I had my own containers, but it was a pain in the rear. I was able to get the Nitrates down to 20 (it started over 160). I also had been lax in completely changing my filter media. I had been rinsing, then replacing the carbon, but not replacing. I know you have many tanks, but if you want to help your fish quickly you can buy water, or get your own RO system ($150 at Home Depot, and I bet I could install one myself). My research led me to believe the only way to reduce Nitrates was to do water changes (not helping you) or use RO water. Regular filters or chemicals don't work. You can buy Nitrate sponge at a place that carries Marine things, but even the LFS people say it won't help much.
My signature only lists 4 tanks I think. I also have a 29 gal and 2- 20 gal. Yes, MTS to the max. It took me two days to get the water all changed.
Good luck
Well for pity sakes. You can ignore that last post because you are having problems with Nitrites, not Nitrates. As someone sais earlier, if you have live plants that have dead leaves etc. that can cause it. One of my tanks did the Nitrite only spike and it was because of the under story with the plants. Once cleaned, with a couple of water changes, the tank was back to normal Good luck again.