you can buy some chlorine tablets (tabletas de cloro) in most Ferreterias and you can have cleaners come do the job for about $500 pesos, but not every 2 months, twice a year should be fine.
I don't remember the exact name of it... but one could buy it at camping and survivals stuff shops. It's a clear liquid with some silver based... "whatever"... sorry, I am not into chemicals.
But silver is an well known old antibacterial.
Just drops of the liquid are said to keep water drinkable with no noticeable change in taste (except Americans will miss the chlorine taste ).