Ok I have three tanks (55g, 2x5g) and one of the 5's and the 55 have a major nitrate spike and I have lost fish in both tanks. The nitrates are up to 80ppm and a 30% water change didn't help much. What do you think could have caused this sudden change? OBTW my last w/c was about two weeks ago and these tanks have been established and stable for two years now, so I don't see how the spike could have happened. And the strange thing is the other fish don't seem bothered, their breathing and eating is normal it just was strange that I lost two fish in the same day from the different tanks.