Does the dust look like particles, or a film?
There are filtration methods and devices that draw surface water through some kind of filter media - those would probably be the best ways to remove stuff on the surface on the tank. You can DIY something (though nowadays it's probably cheaper to just order it from China) - look up "overflow box" (reef tanks use these). Most overflow boxes go to a sump, but yours can just go to a power-head underwater that drives water across the tank (it's just to generate suction through the overflow box).
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