This might surprise you.... tap water is fine! Rain water will need to have CalMag added to it. Fish water is probably somewhere in between, with pluses and minuses... might be inconsistent, contains some nutes but at different levels and in different forms depending upon temperature, fish activity, fish feeding schedules, oxygen in the pond, etc... and that can make it hard to dial in your nutes. That's why adding it to the compost heap is so much easier!!! Rain barrels are cool, just check the ppm and pH and see what they look like. Personally I use plain unadjusted tap water ever since I got my LEAF HUMUS compost /cow manure/ perlite thing going... haven't had a pH problem since. The humus has gret buffering properties!!