Roseman is right, as usual. Very hard (lots of dissolved minerals, usually limestone) tap water will cause the tank pH to jump up when mixing up a fresh tank or adding water in between tank dumps. You can correct your pH downward with "pHDown" solutions.
RO or distilled water, while nice, isn't all that necessary. I've been running tapwater for the last 10 years. Just have to adjust the tank pH downward after topping up with plain water.
pHDown comes in two flavours; a phosphoric acid based solution for use in flowering (the plants love the extra phosphorus) and a nitric acid based pHDown for vegging plants (breaks down into nitrogen). pHDown is one of the very cheapest solutions you'll buy from a hydro shop. May be available at some discount or hardware stores these days, too.
My dehumidifier puts out about 10 litres of totally mineral and chlorine-free distilled water per day. I sure don't pour the stuff out! It gets recycled back into the res tanks.
Incidentally, chlorine in tapwater won't hurt your plants one bit. If anything, chlorinated water is your friend. It will suppress pathogen growth in your nutrient solution. The chlorine evaporates from open containers of water in about 24 hours, so that's about how long the tank's protection from rampant pathogen growth lasts without your help.
After the chlorine is evaporated, you have to suppress pathogens yourself. I use hydrogen peroxide (H2O2), 50% horticultural grade, at 1ml/litre of tank volume every 3-4 days. Don't use H2O2 with organic nutes.