I have a 400 Gallon FOWLR and it has been running flawless for the past 8 years. I used to check all my levels weekly and after the system was up and running for a year, I was able to check monthly and still never had any ammonia, nitrite or nitrate. Years ticked by and I would lose a fish here and there manly my older ones. However, the past week I lost one Tang then the next morning I lost a small angelfish and my mandarin of 7 years. SO I checked my levels and noticed a rise in ammonia...to the level of .5 ppm so after a water change the levels decreased and now back to watching my levels weekly again. Not sure what creeped up the level since my tank is not overstocked and I definitely don't over feed them, but I am back to watching it and trying to diagnose what made my levels rise.