Mine would have to be using tap water in my first reef that 75gallon..I had a bunch of GHA and the tank just didnt look nice.
Oh and the one time my bubble tip decided to take a walk in the middle of the night and find out if he will blend in the power head...yea I woke up to a white cloudy tank...i rushed and did a like 85% water change and luckly i lost nothing other then the nem..so my lesson was ALWAYS foam up the powerhead if i have a nem LOL