Filter socks are pretty easy to change. Hose em off in the driveway, or drop em in the washing machine. I have 4 7x16's, I change em every 2-3 weeks, and they could go longer.
Ultima II will take too much water to back wash and does not polish.
I have this on my 1600 pond heavily stocked and fed mahseers. Pond is gravel bottom with high flow rate of 3000gph. Wash once a week during summer (could go longer). Wash once every two weeks during winter and spring months.