I have recently started keeping fish and I am pretty sure there's something wrong with my tap water. I cycled my 20 gallon a few weeks ago and I added 3 albino cories, 6 rasboras, and my male betta Comet. I checked the water parameters everyday and they were always good:
Water Hardness: 180ppm (highest my test kit went so it could be a little higher)
Though everything on my tests seemed fine, my rasboras started dying. Every 5-7 days I would find one dead. My other fish seemed fine though. I went on a 5 day over Chistmas and when I got back another rasbora died leaving me with 2 and my betta had terrible fin rot.
I checked the water right away and everything was perfectly normal. The only reason I can thing of that could be harming my fish is that there's something bad in my tap water that I'm not testing for. If this is the case, I was thinking that I could use RO water to help lower the levels of whatever is hurting my fish.
I know that you can't use pure RO water, so how much should I use in my tank? Also, where is the most affordable place to buy RO water and about how much does it cost?