safe mosquito repellent?

What's your goal? Are you trying to keep mosquitos out of your house, or out of an outside area? Is your tank inside or outside? If inside, just put screens on windows and doors (which I'm sure you thought of yourself). If outside, there are propane powered mosquito traps that are supposed to be very effective. They burn propane to give off moist warm CO2 (like animals exhale) which attracts mosquitos and then it traps and kills them with a vacuume cleaner or water trap. They are rather expensive, but there may be some do it yourself projects on the internet that you can try. Here's a link to a review of some commercial ones:
 
Most are ok, but try to keep any smoking repellents and aerosols to a minimum in the vicinity of the tank, especially if you have a protein skimmer and/or open-topped tank. I live in mosquito heaven--not as bad as the everglades, but we get desanguinated animals every year.
 
I like turtles too! I laughed so hard I spit coffee on my computer screen. And I know the mosquito's in Ace's neck of the woods are HUGE in the summer.
 
AquariaCentral.com