It's a good idea to keep mosquitoes out by blocking holes with screen-type material. Another way to control them is using Bti in the water -- one commercially available product in the US is Mosquito-bits/dunks but there are others. If you are using a separate larger reservoir with siphon or another way of supplying the container's reservoir, you can put fish in the larger reservoir. I use goldfish and minnows because they are winter hardy here, but they sell "mosquito fish" in the warmer climates.
Algae will grow in the water. Is that what you mean?
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