[RoDi setup and storage?]

I'm in the same boat as Origami. I've been using Rubbermaid Brutes, but am moving over to a pair of polyethylene tanks as well. The only issue that I have ever seen with Brute cans is that some of the ones that have been recently produced seem to leach phosphates into water stored in them. Otherwise, just follow Origami's advice and you'll be good to go.

[De nitrification coil]

Out of curiosity, is there any reason in particular you dislike biopellets? Like vodka, they're just another source of carbon/energy for the beneficial bacteria in the tank to use.

[Low Mg]

It's a bit of an expensive option by comparison, but the Milwaukee digital refractometer is probably one of the easiest to read.