that looks like nute burn of some sort. ease off on the young plant.
btw, tap water is fine. plants do need some cl- ions to function. If you deprive them of that trace element, they will not be as fruitful in the long-run, peronal opinions aside. If it is too high on the water report analysis for your area, you can let it sit for 24 hours. Cl- will evaporate or whatever the proper term is (leaves solution as gas, not a precipitate) at a fast rate, especially if exposed to the sun.