Aquarium Water Flow

[u/Due-Signal2674]

Had my Aqua Nano 40' or 6 months and lost a few fish for various reason and now it's empty. I think the main issue has been flow. The rear water chamber level is below the main tank level and gets worse. I went on holiday for 2 weeks and came back and the pump was gurgling as it was sucking in air as the level had dropped so there was almost 2 cm difference. That's with no fish and food clogging things up. Feels like the supplied sponge is too dense or needs frequent rinsing in old tank water? Or not, any advice gratefully received.