All of this depends on your Water params as well. How is your tap? I would highly recommend doing a test for Nitrates or TDS
on your tap. Depending on where you live, your water could be super gunky, or not that bad.
I too have heard the horror stories of not using RO
water: Algae outbreaks, accumilation of neutralized metals that eventually kill everything, etc. I had a plan to experiment with a 10g saltwater tank, using tap, and just compare a side by side difference.
Assuming you only plan on keeping a FOWLR
, no corals, only plan on keeping 1-2 fish (hopefully hardy fish), do weekly water changes, and not too be too upset if all goes wrong (because we have all heard that tap is a no no). Then go ahead and experiment. Please share if all goes well or bad.