Fishing regulation in Slovenia are regulated with the Freshwater Fisheries Act.
One of the things regulated with this is that every fish that is removed from the water (Catch and take) has to be replaced by the club that manages the water.
Slovenia also has a fishing association (Ribiška Zevza Slovenije) which stands above al the local fishing clubs/families that manage the waters. The fishing families can set the rules for the waters they manage this means some waters are only accessible for fly fishing or maybe only catch and release is allowed in the water you want to fish.
To fish the waters of Slovenia you don’t need a country license but you do need to buy a license of the fishing family. For most of the water you can buy a license via Ribiške Karte but some clubs only offer their license via their own site.