If I’m reading your intent, you want to add tap water to your tank,then dechlorinate,and then add salt? That’s just crazy.

i used to use RO water in my ten gallon FOWLR tank but now i just use treated aged tap water and it works fine.
i would only worry about it if you have corals and inverts.
fish only with live rock will be fine.

Water conditioners will remove chlorine or chloramine (you should contact your water prodider to make certain of which they use and that you’re using the proper conditioner to remove it) and it will also neutralize some metals which may be harmful to your fish.

It DOESN’T remove nitrate or phosphate which is also present in many water supplies (and some providers add the phosphate to help prevent pipe corrosion). Since both nitrate and phosphate are plant (and algae) fertilizers, this can make a huge difference in maintaining your tank. By controlling the nitrate and phosphate, you reduce the possibilty of cyanobacteria, diatoms, and other “nuisance” algaes.

It’s downside is that it also removes some beneficial elements like calcium and magnesium which are used by corals and other inverts. Some is replaced by the salt mix, but if you have organisms that use these heavily, you might need to add a supplement to replace what was lost.

While it’s not absolutely necessary to use RO water for tanks with just fish, or fish and live rock, it is strongly suggested for reef tanks, just to reduce the amount of nitrate and phosphate in the water