If the number of insects in the pool is large, it is important to combat all stages of their development, that is, in addition to adults, with eggs and larvae. Otherwise, all the time, new specimens will hatch, and to get rid of them completely will not work.
There are a lot of means to combat mosquitoes in the pool, the main thing is to choose and use them correctly. Put Bti dna (Bacillus thuringiensis israelensis) in the fountain. This natural bacterium will help repel mosquitoes before they can breed in or near the fountain.