All my heaters are seperate from the filter. What sort of filter did you
buy?

Eheim makes a canister filter with the heater built in. However, I was
unsure whether they can be bought seperately. A poke around the Eheim
section of a aqarium supplies website would likely reveal the answer.

Looking at my 2128, I think it would be difficult to retro-fit the heater.
And Eheim don't sell the heater separately. You'd have to buy the entire
2128 or 2126 canister, plus the thermostat unit. (See
http://www.eheim.de/eheim/pdf/english/ersatzteile/2026b2128%20(GB).pdf)

Cheers,

Michi.