Mosquitoes are small insects, which
wild turtles would feed on. I emphasised wild because I can't confirm for domesticate ones that we keep as a pet. On one site
here, they indicate that:
The red-eared slider turtle is generally thought to be the most voracious turtle that feeds on mosquito larvae.
While the larvae aren't considered harmful, the adult mosquitoes are the ones that you should be worried about. Mosquitoes generally prefer to feed on warm-bloodied mammals like us, but they could potentially feed off from reptiles like your turtles and transfer possible diseases they carry. I'd opt to clean the tank again, and focus on removing any Mosquito pupa in the tank. They look like this, with different shade variants:

If you do find adult mosquitoes already active and flying around in there, having them removed asap is also suggested.