Depends on the quality of your tap water, which varies widely depending on location and time of year. It also depends on the source of tap water, is it a municipal water supply or is it well water? These would all factor in. That's why some people choose to go with R/O water, it eliminates that variable.
No. Brita filters are simple charcoal filters which simply filter out fine particulate matter and some odor, that's about it. You'll still likely have the same mineral salt content as you had with the regular tap water. Most R/O systems use anywhere from 3 to 7 stages of filtration. I have a 5 stage system. First stage is a simple sand filtration, 2nd stage is carbon filtration, the 3rd stage is when the water is actually pushed through the R/O membrane (which is, to put simply, a membrane of plastic that has microscopic pores which are so small, it only allows pure water molecules to penetrate, leaving behind various mineral salts that end up going down the drain, etc.), stage 4 is polishing (whatever that means), and stage 5 is yet another carbon filter.
Yes, this is an option. However the water may contain molds and/or bacteria, so using it in a hydroponics setup isn't advisable, but you could give it a try on a test plant and see how it works for you.
You don't need to calibrate your pH meter every time, your local hydro guy was just trying to scam you into repeatedly buying the calibration solution.
General Hydroponics makes a pH calibration solution- you need to get pH 4 and pH 7 to calibrate your pH meter. I'm not sure about TDS, never used one before.
I calibrate once every 4-6 weeks. I also store my probe in cal solution so when I turn it on to use I can tell if it has shifted or not. Each maker has different prefs on storage.
