Local allies in public health: Fish - Las Vegas Sun: "The newest sign of these economic times can be found hanging on back-yard gates throughout the valley:

“Fish at work.”

It’s public health authorities’ way of alerting neighbors that, yes, the algae-filled swimming pool in the back yard of that foreclosed or abandoned home is a potential breeding ground for mosquitoes — but not to worry, our fish are on the case."
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The foreclosed homes have absolutely disgusting pools (the article has a photo of one in Henderson) that turn into mosquito breeding grounds. The public health officials dump in the mosquito fish and put up a cute sign.

However, the sign means that nobody can put pool cleaning chemicals in the water to get rid of the disgusting mess, because the chemicals will kill the fish. So this solution works fine, except that nobody can clean the pool. I wonder what happens if the HOA has a rule saying you can't have an Amazonian lagoon in your backyard.