Thanks for your quick reply. I really appreciate your comments and expertise.
Yes, I absolutely concur in that chlorinating liquid or bleach is absolutely the first choice (I’ve seen in the guidances) but I find this option really very bother to use. It causes white stains to clothes if not properly handled and it degrades over time if not properly stored. High volume is also a matter of concern because of its low concentration (about 15%). So I’ve discarded this option but I still keep a 10L bottle to use in case of unexpected incidents (fecal?).

So, don't you concur that it is possible to keep the pool clean showing crystal water during the long winter period (actually during almost 8 months that nobody swims in it) with only one Trichlor tablet (alternating with 4-effects tablets) that lasts for more than 1.5 weeks in the skimmer if monthly shocks with cal-hypo are done (the shock allows skipping the use of the Trichlor tablet for some days every month) ?. Although the chlorine level during most part of the year is really very low (< 1ppm, about 0.6 ppm) because the filter pump is running only two hours per day, nobody enters into the swimming pool during 8 months !. So water contamination is also really very low. Water temperature is also low (well below 20 ºC many weeks) and this prevents algae growing.
Of course this should be combined with frequent (weakly) cleanings of the pool to keep it free of leaves and with regular wall brushes and floor vacuums.
During bathing period on summer, chlorine is of course raised above 1 ppm but this is only for 4 months.
I estimate that at least one third of the water is thrown away in a yearly basis through vacuums, cleanings and backwashes. Maybe this is too much water, but perhaps it may allow not exceeding calcium levels of 300 mg/dl and CYAs of 70 ppm.
If cal-hypo shocks are made in a limited way it sounds to me that this allows for a big margin given that each 10 ppm of cal-hypo only adds 7 ppm CH. As pointed out before, part of the water is completely renewed in a yearly basis. Don’t you think?
Thanks for giving the opportunity to discuss this with you and for your time. You must be very busy answering questions because most pool dealers recommendations are really a complete disaster, at least here in Barcelona (Spain)