I'd get yourself an RO system and run your rain water through it......you're already going to need CalMag using the poor rainwater......take the same water and RO it....then use it. You shouldn't need the UV sterilization either but could still use it if you wante too. The trick would be how to feed the rainwater through the system since the ones I've seen are designed to hook up to a pressurized water system to run.

Or go to an RO system for your 8.5 SLUDGE.....but I'd go with as many (is it five?) filtration stages as you possibly can if you use tap water.