If you're cheap, and know where you want the powerhead, you can also just put a dab of silicone on the suction cups, and stick it to the glass underwater. The silicone will cure a bit and keep the maxijet from falling.
Anything held simply by suction cups in salt water will fall off because the plastic hardens. Best to use a little silicone on anything else with a suction cup, like heaters and heater probes.