RO Water - Mix With Tap Or Remineralise?

Like most people living in the South of England, I have hard water. (107 Ca mg/l - 267.5 CaCO3 mg/l)

I really want to keep a shoal of Cardinal Tetras so I'm considering using RO water.

I'm torn between mixing tap water with RO water or just using 100% RO water and remineralising it.

I can get RO water from my LFS quite cheaply so that's not a concern.

Maintaining constant parameters is my only concern and mixing tap water and RO seems a bit hit-and-miss to me.

Is anyone else in the same situation?

Can anyone recommend a good remineralising product I can buy readily in the UK?

Does anyone have any good advice about mixing tap water and RO?

Presumably I'm going to need a kit to test for GH too. Any recommendations for a easy-to-read GH test kit?

Organisms cannot survive in R/O water. The point in removing them is so you can replace them with a mix of minerals more suitable to the species you wish to keep. There is someone here who has talked about mixing R/O and tap water successfully, but I can't remember who it was.

EDIT: After a search I found this

I decided to mix with tap water in order to maintain mineral levels but keep the water soft enough for the Cardinal tetras.

The other point raised in the OP was about a recommendation for a GH test.

I actually plumped for  a TDS meter, cost about £6 on Amazon and its just the job.

It gives an instant reading (in PPM) of Total Dissolved Solids and doubles up as a digital thermometer.