About two days after I exchanged my empty 10 lb CO2 cylinder for a new one three of my dwarf neon rainbow fish died 
and ever since then about every couple of weeks a fish gets sick for no reason and dies. It’s been about two months now and I have tested the water for every thing that I can think of but it’s all fine. The pH only changes from about 7.0 in the morning to about 7.1 / 7.2 in the evening. The temp stays around 79 F. and the ammonia, nitrite, nitrate and phosphate are all at the levels they should be. The only thing I can think of now is that the new CO2 has something in it. Could this be possible? 
I got it at the same local welding shop as the other one.
and ever since then about every couple of weeks a fish gets sick for no reason and dies. It’s been about two months now and I have tested the water for every thing that I can think of but it’s all fine. The pH only changes from about 7.0 in the morning to about 7.1 / 7.2 in the evening. The temp stays around 79 F. and the ammonia, nitrite, nitrate and phosphate are all at the levels they should be. The only thing I can think of now is that the new CO2 has something in it. Could this be possible? I got it at the same local welding shop as the other one.