My fish have been dying off one by one over the past week or so, lost 8 angelfish and two rosy tetras so far, they start to lose their ability to stay upright and then just start floating around then die shortly afterwards. Some of the fish had littles red dots on them and it looks like the base of their fins are red too.. Tank is 7x2.5x2, planted, only has 8 rosy tetras and 3 one inch black acaras in it now. I'm running an FX6 and two APS 2000lph cannisters. 50% water change once a week, did an extra one when the deaths started but didn't help any. Ammonia 0, Nitrite 0, Nitrate 10 - 20. Any help would be great.