I think I have sorted out this leaky problem now. Doing Tetratec's investigative work for them.
The back story is I have had this 'orrible' filter in a cupboard for quite a few years now since I replaced it with an Eheim.
This was probably one of the originals and yes Tetratec sent me a new hose adaptor unit to 'solve the problem' which didn't
I needed an extra filter on one of my tanks and thought I'd give it a bash and yup still leaking. So I took the powerhead casing off, then put it back on the cannister and connected it up and ran it again. This way I can see where the water is coming from............and it was coming from the edge of the prime button. It fills up the powerhead casing and then drips over the edge (where the handles are.)
Unscrewed the prime button (not the casing just the button which is a twistlock) and at the bottom there is a rubber washer with a groove in it. I am not sure whether this groove is there when new or if it is 'worn in'. I turned that washer around and re-seated it and then twistlocked the prime button back on.
I then undid the 5 screws and took the prime casing off. There is a rubber diaphragm that seats in there and you can see the sections of that diaphragm that are in contact with the running water because they are roughened. I took that out, cleaned it and wiped it over with some silicon grease. Then cleaned thoroughly the casing and reseated the disphragm. Then screw the casing back on.
Started it up and there is now only the tiniest leak. No drip and in over an hour it is probably only 2 or 3 ml but in that hour I could see that what was a dry compartment had a tiny bit of water in it. It is only in a section next to the prime button casing so definitely coming from there.
Next I pressed the prime button and the leak got quicker. So I took the casing off again. Dried the diaphragm, dried the casing, greased the diaphragm and put it all back together again. This time I didn't press the prime button at all, I left it well alone and the filter has been running now for over 2 hours. Still that 2 or 3ml leak over an hour but it is easily usable now.
So far from it being the hose adaptor or O rings which Tetratec always tell us it seems to me that it is more than likely the dodgy diaphragm design that the prime button uses.
I ma trying to figure out a way of blocking off the prime button entry now as it is right above and open channel which is the inflow to the filter. If I can seal the top of that channel off then there will be no more leaks
I also notice someone above talking about a noisy impellor which was replaced with no improvement. I have that too and I bought a new impellor years ago thinking it would stop the noise.
The only thing I can think of (and its a guess) is that air gets into this filter the same way that the water leaks out and that air means over time the impellor damages its housing.
So this filter is awesome but do not use the prime button and if it starts leaking look there before you waste time and/or money on O rings or hose adaptors
I will report back if it starts leaking again and if I think of a way to seal the top of that channel.