Mine does that and what i do is i wet the seal with water prior to closing the lid and the leak eventually goes away. I know i have to replace the gasket pretty soon but it works for me at the moment.
i have one of my tanks a 125 with 2 fluval 403's packed with 100 bucks each of ehiem substrate and purigen. hahahaha they are both wrapped with bungeee cords and taped all the way around, they have been pumping for awhile it takes forever to clean them but its been working, i have lots of baasteria in them the nitites read 5 every month hahahaha
My eheim 2028 started leaking, and I bout lost my mind "adjusting gaskets.....cutting and clamping hoses........... cleaning impellers....water everywhere..........) well it turned out my "double tap conector valve) was creacked at it's weakest point............The neck of the connection valve.............well big als sent me a new one free----great company by the way---- and i am back in action.................so my point is check your connection / shutoff valves............i could barely see the crack .....but believe, me check closely and you might be suprised.................good luck!!
I used to keep my fluval in a bucket for that same reason. I like the internal fluvals, but the external ones almost always leak. See if they have a warranty or if the store will take it back. Even though you've had it for a long time there's always a possability they may decide to be nice and replace or refund it.