Your tank is small and over stocked.Your tap water is almost neutral with very little bicarbonates at 7.3.The food and their waste are depleting the water quicker in a small tank,lowering the PH.Plus as mentioned the tank is not cycled yet.Ammonia in water below neutral (7) shouldn't be toxic to your fish,but when you change your water it brings it back to the range where it could be harmful.I see what looks like a bit of an ammonia burn on the fins.