as the fish shop they should have pond clarifiers or UV set-ups for aquariums. the super expensive (and probably huge) stuff you probably are looking at is for drinking water production (that is for sterilizing water i think)
In fact i found that Bazoka size UV system in a fish shop, Maybe i'll get a water filter system - for drinking water- and run the tank water throw it before going into the tank filter i guess this will be less expensive