So to confirm, I can just connect this to my tap when I'm using it?
Yeah. It comes with an end piece like a hose uses to screw into a spigot. You unscrew the tip of your faucet where the screen is, bring that to the store to find the matching adapter for a hose screw, and screw that onto the end of the filter hose. Then when you want to make water you unscrew the faucet screen and screw on the adapter with the rodi attached to it.
The adapters like $3
I used tap water for a long time, and it was fine.
But the thing is you never know what's in it. Stuff like copper that is fine for us can be a disaster for coral, and phosphates etc can cause algae. So anytime your tank acts a little mad you wonder if there's something bad you can't test for in the water.
Also, the salts we use are very specifically balanced for minerals and alkalinity. They are just like sea water with only the h2o removed. So if your tap water is a little hard, or has boron or whatev in it to start with, you aren't getting the right balance out of your salt. The rodi gives you just the h2o.
And, because of the evaporation you add more water over time than just what you swap out in water changes. So you can have stuff build up and in the case of copper the rocks will absorb it and cause a prob later even after you switch to pure.
I'm glad I got the filter, it feels like a luxury not to have to worry.