I recently moved in with my girlfriend and for a couple of reasons I decided to buy a new tank (325 litres in stead of 200 litres). Because the existing filter has enough capacity i wanted to keep using that one and place it on the new tank. The only problem is that my fish should still stay in the old tank until the new aquarium was cycled and all. So i planned to use my really old filter that i had in storage for my old tank until the fishes are all relocated.
All well and fine until i discovered that the rubber to keep the filter watertight is missing this evening

. I am not eager to buy a filter for this purpose only and i am a bit at a loss what to do now? Can i keep the fishes alive in the old tank with some aeration by an airpump and regular waterchanges or should i plan for a more creative solution?
Ow, and i have a bout a month and a half before i have to remove the tank from my old house.
Tank stocking for extra info:
3 Synodontis mbandata
1 Synodontis nigriventis
3 Ctenopoma weeksi
3 Pantodon bucholzi
6 Alestopetersius caudalis
Thank you in advance