Yes a Reverse Osmosis filter is a good option.
Depending on how hard the water is and what species you have, they may be able to adapt fine.
The hard water has minerals (mostly calcium) dissolved in it. To make it 'soft' you have to somehow remove these minerals.
Another option is to catch rainwater (VERY soft) from your roof, and mix that with the hard wellwater to get some medium water.
4th option is to specialise in African Rift Lake chiclids and cats. They wil love the water.
Cheers
Ian