What would your goal be for that? For most FW fish, you would just have to add some dissolved solids back into the water. If you are importing South American fish, I could see using a mix of RO and tapwater to get the DS levels down to where the fish will do better - but straight RO water is bad for most/all FW species.
I've always found that tapwater is generally best - even the worst source water can be massaged into use for FW fish easier than RO. This assumes that the tapwater is safe for human consumption.
Chlorine can be removed by carbon or dechlorinators, the pH can be adjusted as needed, and mixing a percentage of RO water in can adjust the hardness if needed. There is some well water that is high in sulphur, and that needs to be dealt with through aeration or hydrogen peroxide. Point wells can also be contaminated with bacteria and farm chemicals.
I use poly tanks from US Plastics as mixing vats for FW and SW....
In my store we use the rejection water from the RO/DI system for our FW systems. TDS comes out at about 100 to 125 on average. The wasted water from the RO/DI is run to a res. and used for other things too like the washing machine and deep sink. Works very well for our FW stock.
I think that's the best way to go. Can even set it up for basically a constant water change system if you can keep it warm enough on a large system. I think that we are lucky to have our rejection TDS come out just about right