I had 11 mollies 3 corys,and seven Neon tetras in my 32 gallon aquacube. Last Saturday the 20th I added 5 neon tetras. Since then I had 5 mollies and 9 tetras die. (5 original tetras). The tetras all died within 2 days, the mollies one a day. My water test shows no issues with Nitrate, Nitrites, ammonia, or ph. I have been treating the aquarium with herbtana. Other than the water change I am currently doing is there something else seem to be going on that I'm unaware of?