Thanks for the suggestions. The ball valve is a good idea to keep any floaties out, even though they'd be trapped by the sand. Same with the heavy metals. The system is basically a condensed version of a natural spring.

The chemicals are decomposed by the biofilm layer. Check Susan's links from the previous thread for more information. Supposedly they are very effective.

Mosquito-proof screens will cover everything! We have a lot of cases of West Nile Disease here.

-Blast