U use the Tunze on my 30G and love it - Has two speeds, very quite. The air is sucked up from top and funneled out, really good design
http://www.marinedepot.com/Tunze_Aquawind_Aquarium_Fan_Cooling_Fans_for_Aquarium_Lighting-Tunze-TZ8611-FILTACAF-vi.html
but if you looking to save dough, you...