You will need an overflow on the sump. Get a carbon filter for your water feed, you will have to watch how many gallons it's good for, and change it regularly. Get an electrical water valve, the same as used for sprinkler systems, and a timer. Hook this in line with the carbon filter. Determining how long to run it for will take a little trial & error, along with a valve inline to adjust the flow. If you can figure out how many gph with a 5 gallon bucket, you will know how long to set the timer for. Make sure the overflow can handle the water input rate.
You could also use drip emitters for a constant supply of fresh water, these come standard as 1/2, 1, & 2 gph. They are used for irrigating plants, but work just as well for aquariums.
I just got off the phone with a local aquarist who has his setup running the same as the first option. He also set up a bypass for the timer with a second valve, and a remote control designed for lights. He has a couple dozen 10's & 20's, an overflow on each, and a small line running from pvc to each tank. He can watch tv, push a button, and the water starts. It took some time to set up, but I've never seen an easier way of doing water changes.