To keep your water nice you need to know water chemistry. Relying on someone else is a disaster waiting to happen IMO.
You need to decide what you are going to use - here in the US and Canada we have chlorine and bromine - do you have them in Spain? Usually when we mention chlorine there are 2 types we are talking about - dichor (which has a longer name) and lithium chloride. Bromine comes in 2 forms here - powder that is straight bromine and pucks which are a combo of bromine and chlorine.
You have to keep your PH - how acidic or alkaline your water is - in the range of 7.2 to 7.8.
Alkalinity - this keeps your PH stabile - in the range of 80 to 120.
Calcium - this is for water hardness - minimum 150 to a max of about 350. Anything below 150 and the water can foam.
Keep body products, hair products and whatever else might be on your body to a minimum. Don't wash the bathing suit, if you must rinse the heck out of it.
I think I covered the basics. Any other questions, just ask!