[quote=if reverse osmosis is used in residential use then why do people do this themselves? QUOTE]
Because it's not at the municipal level, the units are added privately.
If you can get good enough water out of the tap then great! Just keep in mind that water quality can swing greatly with winter run off, heavy rains carrying pesticides and extra chemicals added to the water supply to combat the poor water. If you aren't having any phosphate problems at the tap source then maybe your algae problems are being caused by excess feeding. I would run a few tests before buying an RO unit, although you
can get a descent RO/DI set-up for around $200 new.