Hi Ellie,
yes, you can use tap water. Just bring it up to room temperature and if you like, let it set for overnight to let any Chlorine dissipate. I personally use sometimes tap water, but prolonged use of the hard water in my area clogs up the "pores"/stomata, and some delicate Orchid suffer in the long run. I use mostly rainwater. Don't bother with distilled water, it has no benefit at all.
I kept Fish in my greenhouse for some years and Orchids absolutely loved the "dirty" water I poured over them when I cleaned the tanks. Especially small mounted species and Slipper Orchids really thrived on that routine.
You might add some fertilizer with a higher middle number once in a while to regular water as a bloom booster.
