Having had an Intex 15' donut for 3 years I can assure that if you keep your water properly maintained the filter should be sufficient. But I can also assure you that if you run into problems you will be washing down that cartridge every 24 hours and it STILL may take weeks to clear up. About the only thing you can do with confidence is sanitize the pool with lots and lots of chlorine--but clearing cloudy water or constant precipitation? VERY hard to do.

But if it's sanitary and you check your chlorine levels every single day, you CAN keep it safe enough to swim in.