Yes, Grand Funk Overdrive is exactly it
No its Grannular Ferric Oxide and it is an effective phospate binding media.
The reason you want to use RO/Di is because tap water will likely contain higher concentrations of nutrients you don't want (phosphates, nitrates, etc.) as well as the potential to contain all sorts of other things we don't have test kits for and you have no control over once in your system. Your rock will essentially serve as a sponge for these nutrients and while you may not see a ton of nuissance algae in an unlit tub, once you add it to a lighted tank I promise you that you'll wish you didn't use tap water.
The reason the ocean doesn't need a RO/DI unit is due to dilution which as some say "is the solution to pollution". The other reason would be the variety of life that will consume those nutrients. No way you can compare even some of the largest home tanks to the ocean in terms of filtration. I have read where people ask why a skimmer is necessary because the ocean doesn't have one but in actuality it does. That's what beaches are for
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Not saying you can't use tap but you are opening up youself to a lot of unknowns should you have issues with algae, fish deaths, etc. once you get to stocking your display.