Britinusa, how can you 'tell' that the cartridge from last season is 'working'? When I got my pool it came with a Nature2 system and I could never figure out if it was working or not... There was no 'test' that came with it to tell me what it was doing or not doing, so after a few months I just took it out and haven't bought any replacements (don't intend to either, after I found this place and learned that it was putting metals into my pool! ). I didn't have any problems with it (staining, green hair, etc), I just can't see how it could be a good thing to load up the water with copper and silver...