You'll have to ask all the Eheim lovers that think there the best filters out there and say they never break like Tk1armann. But if I would take a guess it most likely needs a new seal. You can sell it and buy two new fx5's fully loaded.
Have you checked to see if the o-ring seal is located in the groove of the lid? If this is an older filter, it may be time to purchase a new one as they tend to compress over time.
There is no need to use any tools to tighten fittings or lid locks on any brand of canister filter. Hopefully you did not distort the plastic lid by using a ratchet
May be time for a new seal, but try to reset it again, and dont use a ratchet hand tight or should I say strong finger tight works. When tighteneing use a star like pattern makeing sure even tightning all around...
I must have stupid big fingers but I couldnt tighten the nuts enough with my fingers. Does the grey cover have to me on? If not I would be able to do that.
No grey lid dosnt have to be on, reset the o ring on the filter but before doing that run a little bit of Vaseline on the rubber o ring, normally works a treat