Put a fishtank aerator in it. The oxygen in the water will keep it clear, kill anaerobic pathogen bacteria and the bubbles disturbing the surface will keep mosquitoes from breeding in it.

Standing stagnant water can harbor legionnaire's disease, be a mosquito breeding ground, etc. And as @mike says, regional hazards vary, check with your local health authority, what works here in Oregon might not work for example, in Florida, etc.