I have a 12g nano tank with 2 18w cpf bulbs in a cannopy that came with the tank, well every time i turn on my lights the fans make this horrable buzzing noise like they are grinding on somthing but theire not???im not really sure what is wrong with them. i tried finding replacement fans but i cannot find the right size needed for my tank. Every time this happens i smack my hood and it stops for a couple of seconds then starts again.what is wrong with them, and how can i fix this????