I agree with batman 100%. But also, you could try some trapdoor snails.I have about 20 in my outdoor trough.I live in Devore,CA. Witch is 5 miles north of San Bernardino, or 50 miles southeast from Los Angeles. It gets about 35 degrees here at night in the winter, and the snails are FAT AND HAPPY!!!. they are unharmed by the cold. I was almost sertain that they would perish with the temp at night. But they surprised me by not only surviving, but reproducing little bitty snails all over the trough.In this 35 degree weather, the water temp is around 39-44 degrees without a heater.
To use these snails, you need 5-8 snails per square foot i think.If someone sees this and can correct me,please do.. :)
just an idea