sicktetra

Hi guys, can someone please help me. I have a 95l tank planted tank with 4 cardinals, 5 emperor tetras, 4 rummynose tetras, 6 harlequins, 4 corydoras and a pleco. I do partial water changes 2-3 times a week. Water parameters are 0 ammonia 0 nitrites 10-20 nitrates. 4 of my emperor tetras have...