newbie - Is RO water really necessary?

I'm a newbie setting up a 55 gal fish only aquarium. I have been
planning to do a freshwater setup, but realized lately that the most
colorful and interesting fish IMO are sal****er. So, I've been looking
at sal****er instead. Still fish only, and flake eating fish only if
possible. Everything looks doable for a newbie like me, not too much
more than fresh... except reverse osmosis / distilled water.

Is that RO/distilled really necessary? That would add substantially
more $$$ for those that don't have a RO setup, like me.

Thanks in advance