I had a professional put a pond in my background. By my estimates, it's about 600 gallons, concrete basin. The pump and filter are one unit, inside a white bucket, with a PVC pipe connected to the outflow going to a waterfall.

It's time to change the filter and I'm wondering what I should put in it? I've been calling the guy who put it in and have received no responses, which is very frustating. I realize I can just take it apart and look, but I'd rather have an idea of what I need first, so I have the supplies onhand. Any suggestions appreciated.