I have a 75g that I started a couple of months ago. Started tank with Carbsea Ocean Direct Live Sand as well with one of the bacteria in a bottle. Cycled the tank for two weeks with a couple of Mollies also in the tank. Everything was going good after the tests showed my Ammonia, Nitrite, and Nitrate good. Around the 1 month mark I added a Racoon Butterfly, Coral Beauty Angelfish, Clown fish, and a school of Athinas. After 2 weeks I started to notice some skin issues on a couple of the Athinas. I have Hanna testers for everything but Ammonia and Nitrite. I still just had API for those two. I tested my water and the Ammonia showed between 0-0.25ppm but for Nitrite it was at like 5PPM. According to my Hanna my Nitrate was at 10ppm. I started to do 25% water changes every other day for a week. During that time 2 of my Athina's died. So I decided to pull all the fish except the Mollies and put them in one of my 40g QT tanks. I spent two weeks doing water changes of 15-20g WC every other day. After the 2 weeks the numbers all stabilized at zero. During that time the fish that had nitrite burns scales healed. I moved the fish back into the tank and after about a week I started to notice the irritation on skin returning and I checked my numbers and I was back up over 1ppm Nitrite. Ammonia was 0.25 Nitrate around 10. For the last 2 weeks now I have been doing daily 15g water changes and nothing I can do will keep the Nitrite numbers bellow 0.5ppm. When I moved all the fish before I moved all my rocks looked for any dead fish or anything but could not find any. So I can't figure out what could possibly be causing my Nitrite numbers being so out of control. Any one have any ideas or advice on things to try? I added 1liter of Seachem Matrix to the sump to help give more places for bacteria to live added Purigen as well. Already had GFO running from day one on the tank so its levels never went up. I have less fish in this 75g that has the issues than my other 75s or even my 2 40g's. I am not over feeding. I just can't think what could be wrong.