Bottled water.
I have fish tanks (one regular, one RO), and I buy RO water and distilled water (yes, I know a tankful of distilled water is not good for fish, but diluting my very hard, alkali tapwater with it 4:1 keeps the tank in good parameters, and does not hurt the fish with biweekly water changes of 5gal in a 40gal tank). The drinking water specifically at Walmart is perfect for an RO fish tank. It is extremely soft, because they don’t add anything back in, and the pH is right for my tank. The drinking water at other places doesn’t work. I have been having trouble getting water for my fish.
I bought a Britta filter, and have been running gallons of water from the RO tank through it, and using chemicals that are supposed to remove ammonia, using more distilled water, bought where and when I can find it, than I am really comfortable with, and occasional tapwater. So far, the RO tank is pretty well in parameters, and I have not lost a fish that I know of, but it’s the tank where my Panda Cories breed, and I have not had many babies. I don’t know if they have been breeding less or if the eggs or fry are not surviving. When they are about 1/2 grown, I take them to one of the stores in town, and get credit towards food and stuff. Apparently my Panda Cories are pretty hardy, and customers like them.
I tried buying an RO filter years ago, and could never get it to work properly. Paying ~$100 for a filter, and never buying RO water again seemed ideal, but I tried for weeks, getting various hoses and bottles and things at Home Depot, and could never get a rig that worked well.
I went to one of the Walmarts in town, and the water shelf had one, sad, misshapen bottle. I actually ordered from Walmart online, because the online service could get me water. I was limited to a certain number of bottles, but I got enough for next week’s water changes.
I’ve been stretching the changes-- instead of two per week, I started doing them every four days, then four days/five days. The tanks seem OK, and I haven’t lost a fish.
I still worry.