I have a 2500 gal pond and use a " Bead filter by Aquadyne " . IMO you can not beat this filter. You can attach a UV system which I have done and It keeps the water crystal clear. Depending on your fish load you backwash it every 10 days. ( 5 min. ) My koi are huge also and they love this filter. Customer service is in Ga. and they are very nice. Worth looking into . Good Luck