water source

Reverse osmosis water is most likely what you're getting at the store and is not bad for fish at all, some people prefer it. I really doubt a vending machine at a store is going to be dispensing distilled water. Distilling water requires boiling it and allowing the steam to condense into droplets, and that is what is collected. Distilled water isn't good for keeping fish in as it is devoid of the minerals fish need from the water they live in. As far as what is better water, R/O or conditioned tap water, well that depends. Let's say you live in an area like I did in Michigan where the water is extremely hard, on the order of 350+ ppm. If you wanted to keep African cichlids or mollies, fish that like hard water, you could use a mix of tap water and a little R/O to bring the hardness into the proper range and all is well. But if you want to keep discus or another fish that needs very soft water, you might want to put all R/O water in. Test your tap water to see how hard it is and go from there. If it's just right for the fish you want to keep, then conditioned tap water will do fine and it's a lot less work and $$$ than getting the R/O water from the store. If not, you could try a water softener pillow from Aquarium Pharmaceuticals to soften it. It goes right in the filter and isn't hard to use. I used them in Michigan and it worked quite well. They are reusable and not expensive. If you really want all R/O water, maybe think about springing for a small R/O unit for under the sink you use to fill your tank and save the trips to the store and hauling all that water. With a 55 gallon tank, you'll be muling that water around a lot. Hope that helps!