I'm about to buy a small property with a stream fed/rainwater catchment system and know some of the problems with these kinds of systems are the potential for mosquitoes and other waterborne critters to get into the water and breed.
One brilliant idea I came across on the Ludlow Survivors Group was so simple, yet, so perfect, I had to share it here:
If you have problems with mosquitoes or other airborne nasties using the water stored for a solar shower and the toilets they can breed to highly irritating levels with accompanying medical problems.
due to the nature of a lot of these systems you may not be able to seal the containers adequately to prevent these nasties from getting in and laying their eggs, but the problem can quite easily be overcome with the addition of polystyrene balls (such as those used in bean bags) in a thick layer over the surface of the water.
This will prevent the mosquitoes and other nasties from accessing the water whist allowing the system to continue to operate as the balls float on the surface. They also conform to the contours of the storage vessel and can be prevented from accidentally accessing the system when drained by a simple mesh at the inlet. Water added to the system will simply push the balls out of the way and then they instantly return to cover it again.
The suggestion was made by Stuart at BCUK.