I'd unplug the heater and inspect for damage. Cracks could mean you have an electrical current in your tank which can kill fish pretty quickly. I've had heater glass crack after being exposed to air for like a minute. You can also test for stray voltage in the tank if you have a voltmeter (there are instructions on how to do this on Youtube).
Also, what are you using for filter media? I'm not familiar with that tank, but you should probably be able to ditch the sponge and use other bio media, rock rubble, etc. The sponge can really limit water flow through chambers (which is why your water level in the heater/pump chamber is so low I'm guessing).