AquaNano, Fish Keep Getting Stuck In Pump/heater Compartment?

nearly every day I have to rescue my male rainbow killi and his wife from the pump/heater compartment? None of the others get in there and i cant see how they are doing it? Ive put a bit of plastic freezer bag all the way along the back wall but still they get in there, Baffled!

no gap on the filter outlet and ive sealed off the gap with the plastic bag, i cant see anywhere they could be getting through other than jumping a sliding under the bag but that doesn't really seem feasible even for these acrobatic fish

I wouldn't put anything past fish.
Years ago I had some zebra danios. The filter in the tank was a wide rectangular shaped one which had a dimple in the top to allow a finger to prise the front panel off for cleaning. It was situated with just an eighth of an inch of water above it. The danios would wriggle onto the top of the filter and spawn, leaving the eggs in the dimple. If I hadn't actually seen them doing it I wouldn't have believed a fish would actually slither along in such a tiny amount of water and then to spawn there 
The point of this tale is to show that fish will squeeze into the tiniest space if they decide that's what they want to do, and there is not much we can do about it unless we catch them in the act so we know exactly what they are doing, I'm afraid.

I had a ninja glowlight that I regularly had to free from a tiny space between the tank and the back of the filter, I stuffed the gap with wool in the end, I used to be so worried that I'd squish it trying to free it and I still have nightmares about the otto that managed to wedge itself between 2 wooden prongs on a lump of bogwood. I had to leave the house while my son freed it. He had to push down and backwards to free the damned thing, I was sure it would be damaged in some way. After that Dan got a saw out of the shed and sawed off all the bits he thought likely to cause the same problem...off the bogwood not the otto I hasten to add!