is the tank a brand name one so we can find the instructions/specs for it?
is there a sump, a separate tank/container below the tank that acts as a filter or is everything in that compartment in the back?
Have you taken out the thing with the airline attached to it?
just try blowing through the airline and see what happens. that will probably give you an idea of what it's for.
if it doesn't seem to do anything and if it isn't hooked up to anything, no need for it to be there.
IF all the filter materials are in that compartment in the back, what is most likely happening is that the filter pads/media is clogging up restricting the flow of water to the bottom of it where the return pump is located, so it is draining the compartment dry and sucking air before the compartment can fill back up.
remove everything from the back compartment, rinse it lightly in some tank water ( you may as well do a partial water change while your at it) and reassemble the stuff in the order it was.
That should remedy the issue if the filter is the type I'm thinking.
Hope that helps.