Using Tap Water
"Treated" (I use Seachem's "Safe") tap water is fine for the vast majority of fish you get at the pet store, they'll adapt. They have for decades. pH, hardness and all the other chemicals that make up the tap water aren't important to a healthy tank. Changing out a lot of tank water and doing it often is important. Small tanks need large water changes every few days to keep the dissolved fish wastes out of the tank. The longer the old water stays in the tank, the worse the environment for the fish and plants.
Work up to the point you're changing half the water a couple of times a week and your fish and plants will be healthy. Consider a much larger tank. They're more forgiving of mistakes in tank management.
"Fear not my child, just change the tank water."