i have an ibc 1000ltr tank, just bought the 27" immersion heater but will only fill it to about 350-400 ltrs of water depending on what work i have on particular days. If im correct and yours is baffled like the one i used to have a 11" immersion is prob more suitable. Somebody who has the same as you could prob advise more. I bought both falnge and immersion from my local toolstation!
Regards craig