I had a school of 6-7 dwarf rainbows. I have 3 males and have had as many as 4 females. 2 of the original 3 females died and I’ve now lost a total of 4 females in the past month. The latest was found float this morning. Nothing to show any trauma or signs of disease. Everyone else in the tank seems happy but the females in question(one living one showing similar behavior oic attached) never really got with the group and spent the majority of their time at the surface just sitting there. Water is 79-80 degrees 0 ammonia, 0 nitrites, 5-10 nitrates, GH and KH read 0, pH between 6.5 and 7 per my API 5 in 1 strips (brand new set good to 11/2020). Any thoughts?