They breed in stagnant water. If your fountain is large enough and if it worries you, consider getting some small goldfish for it; goldfish/coy gobble up mosquito larva like treats placed especially for them.
From what I've read and been told by people who know, mosquitoes can't lay their eggs in moving water.
I am no expert onmosquitos but I seem to remember they only breed in still water. That is why it is recommended to make sure you have nothing that collects water around your house.
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