bacteria bloom

I have a 240DT in a total 300g FOWLR. I started adding 7ml nopox 2 months back to control nitrates (my phosphate is near-0, I use gfo) but nitrate climb to 50 in 1 week after water change (30%) ..so i up the nopox to 6 ml noon and 6 ml night for last 3 weeks. Any suggestions on what this is ans...