Don't forget that a critically important part of what is taught here is understanding and taking control of your pool. That usually requires a proper test kit and we recommend the Taylor K-2006 (see this sticky post for more info). Unfortunately, you may not be able to get that in Spain. The next best kit you can probably get in Europe is the Palintest SP 315C though ideally also the Palintest FAS-DPD kit as well.

You can certainly use Trichlor or Cal-Hypo if you know what you are doing and understand the consequences. Between the two, if your CH isn't high already then Cal-Hypo may be a better choice. The downside is that you have to add chlorine to the pool every day or two since the typical outdoor pool exposed to sunlight uses around 2 ppm FC per day. Of course, this will depend on the weather in your area and also on pool temperature (some chlorine is broken down from sunlight while some is from reacting with organics in the water and with pool covers). The purpose of these sites is to give you the knowledge to be able to control your pool and not let it control you. The key is to maintain an appropriate FC level for your CYA level since that will properly disinfect the pool and prevent algae growth.