Tap water can be fine aside from the fact of Chlorine and other such chemicals in the water, you can have Calcium in the water which will freeze the uptake of Magnesium and maybe some other vital nutes. You can eliminate the chlorine problem by just letting your water stand with no lid overnight (it will evaporate) or boil it, BUT it does not remove impurities in the water like calcium , which the plant does like, but it's easy to overdose it... so you need a filter for that, because I don't think it boils out... Other then that you need to find out what's in YOUR local water, and it's pH because a high pH means dead plants, kiddo.