Back siphoning with canister filter

[codger]

I have an external filter, a Eheim Professional 2228, servicing a 200 litre tank (well short of the max 600l) and it's been in place and working well for upwards of ten years.

During a recent power-cut, the canister leaked and would have emptied the tank by back-siphoning - fortunately I was present. Previously there was no problem.

What could have caused this?

As I understand it, in practice siphons depend only on the fall from tank-level to outlet-exit (atmospheric pressure etc. are effective constants). Nothing significant has changed, certainly not the position of the filter, or inflow and outflow. One possibility is that the seal between the pump-part of the filter and the container has worn down and now simply leaks. There is no gasket and the seal is just plastic to plastic, four clips press the two surfaces together.

Is it possible that the (rare) cleanings of the filter have eroded the edges and the canister no longer seals?

Can I create/purchase a gasket to fix this?

Is something else going on?

As an interim solution I've lifted the filter onto a table beside not under the tank; this has reduced the siphon pressure and it is now not enough to cause the canister to leak.

[plaalye]

Are you sure there was never an o-ring/gasket that fit between the top & cannister? My classic has one, never owned a pro. Check your manual or go to eheim website for parts list.

[codger]

Ah, yes, odd you should say that..

In the interim I downloaded the manual (http://www.eheim.de/eheim/pdf/en/anleit ... 6_2228.pdf) and spotted a gasket I'd never noticed as it fits inside the top pump-part of the filter.

It's quite visible in http://www.eheimparts.co.uk/eheimparts_2228.htm and I've searched and found other Eheim owners with the same problem. I can't complain - the gasket has done good service and I'll get a replacement.

Thanks for your answer - if I hadn't kept looking I certainly would have done after your reply.