External filters, what we call canisters in the US, are definitely bigger and hold far more media than any internal. You may well do fine with your internal but an external will do better if you find yourself with a marginally acceptable internal filter. I have one of the old Fluval 4+ filters on a 40 gallon long and it does fine for my breeding colony of Ameca splendens but it is no way as robust as the Rena XP2 that I have on another tank of similar size that holds my colony of C. encaustus. The Rena is twice the filter of the other one and the Fluval is about as big as they make for an internal. Fluival also make much larger filters but I have not seen them make a larger one as an internal.