Hi,
Right how to explain this lol.
First of all i have a RO unit hooked up to my water supply, this stops the need for always having to treat your water(imo), so in the long run works out cheap for you.
The water inlet goes into my tank, at a slow trickle. approx 75lts every 24 hours.
The outlet is in the sump linked to my main waste pipe for my house.
I drilled a hole(at the same level as your water line) in the section of the sump where the water level rises and falls. I used a small tank connecter attached to a pipe to go into the waste pipe. This works really well as, when the water rises to right level it goes down the pipe 24/7. it also work if you ever have a power cut your sump will never fill above the outlet.
I have only cleaned my sump once in over a year, water and tank are always clean, fish happy as hell and never have to do any water changes.
I have 18 RBPs in my tank so as you will know not the cleaness of fish. lol
If you have any questions just ask. HTH
