DWC water and nute questions.

Hey guys I just have a couple quick questions.

What kind of water should I use for my 5 gal res? I don't want to use tap water because the PH constanly rises and I'll run out of ph down in a matter of days.
I've been using purified drinking water to wet down the rockwool and to water my soil plant so should I just use that?

Also, I'm using technaflora BC nutes(recipe for success) and it says to change the res weekly. Is this really necessary? I'm only using a 5 gal bucket with one plant. If not then how often should I change it?

Any help is appreciated.

Thanks,
GTS

r o water is the best you can buy a ro water unit for around 150$

RO water is the best to use for all plant because you know exactly what you are giving your plants. Also with DWC you really have to stay on top of the Ph so your better off using the best water possible to illiminate the headache of having to ph your res. every couple of hours.
Doing a water allows you to replace any used nutrients without having the build up of the unused nutrients. If you don't do regular water changes your ppm will skyrocket.