As a volume you should be looking at 2500gallons for the cat and to filter it you need to be looking at some large bay filters,I'm not sure whats availible to you over there but you should look into good quality koi pond filters which will do the job for you, don't expect to do this cheaply though.
when feeding any catfish pellet for a large cat you need to watch for polution. like suggested the trout pellets being very oily a handfull of them will easily pollute and force a water change in even 5ft tanks.
i suggest feeding meaty foods such as lance fish, prawns, mussels, squid, large fish pieces (trout, salmon etc)
you don't really have mouch time before your cat will need to be moved into much large enviroment.