I set up a new tank (20g) about a month ago and have been letting it cycle since. I went to test the water and all of the parameters for adding fish seemed to be fine. The same day I decided to add 4 neon tetra (I know neon tetra are delicate, but I couldn't find another fish I liked at that point). The Next day, one of the neon tetras were dead. The rest of the neons were fine and swimming happily above their moss tree. I got another neon the next day and again, it died. I have not seen any symptoms in any of the other tetras or the two that died. Does anyone have any idea what could have caused this?