The aquarium pharmaceuticals filter will get you better water than a filter designed for drinking water (Pur / Brita). The problem is with replacement depending on the quality of your local water. I had to replace the filter every 20 gallons, which made it really expensive to operate. It actually was cheaper for me to buy distilled water than to run the filter. If you get 40 gallons out of it, that is still about $.50 per gallon. An RO/DI unit will produce more water than that, and will pay for itself faster, in addition to producing higher quality water.
Most of the waste from RO units appears to be running the water through them at too much pressure. Since the unit can't keep up, it rejects more. I get about a 1 to 1 ratio. Right now, since the weather is nice, the rejected water is dropped into another container and is used to water plants outside.
Using well water (especially in PA after the last several years of little rainfall) is a different problem. Distilled is probably a better way to go if you can't buy RO in your area.