Sounds like you're stepping in the same hole all of us have stepped in before - over feeding. When I first started reef keeping, I fed enough that my cleanup crew could come out and have a whole meal themselves which was way too much.
Luckily, your issue is easily solved.
1. Cut back your feeding to 1 cube of mysis/day. Then alternate between algae on one day, mysis on the other. You probably could feed less than 1 cube a day, but I'd have to know what your bio load (fish/inverts, etc) are before I could say for sure.
2. Keep up your current water change amount and schedule for now, you'll need to drop your nitrates to an acceptable level. Once you adjust your feeding schedule, and after about a month, you can back off your water change amount to the normal 10% every 2 weeks as you'll be removing the source of your nitrate issue - too much food.
Common mistake, easy fix.
And certainly having your own RO/DI system will be of benefit to you!