Just come across this about the leaking problems with the Tetratec range of external filters.
Tetra have identified a problem with this range of filters. The hose adaptor unit ball valve spindles can leak water, which then fills up the compartments within the top of the filter and then overflows down the sides of the canister. This leads the customer to think that the main canister seal is leaking, when in fact it is most unlikely.
Tetra have modified the adaptor unit and are fitting it to all new production stock
However, in the meantime, if you experiance such a leakage problem with your existing Tetra Tec EX filter, then you are advised to contact Tetra direct on 08700 55 4031 who will be happy to deal with your problem, generally at no cost to you