i'm no expert on crocs (or any herps for that matter), but i do have 5+ years of keeping large and predatory aquatic fish under my belt so i will tell you this- buy the best filter you can buy! for a 55g i would personally use 2 HOB (hang on back) filters rated for a 55 tank, but since you will not have the water filled all the way you could probably have just one. you'll probably want to change the water often if you plan to have fish in there....
i would personally use an FX5 as filtration for his larger enclosures. also,i don't know what you plan is for your bigger and/or final enclosures but if you intend on keeping fish in there for any duration of time for him to eat you'll want to read up on aquarium cylcing, especially if he doesn't eat them immediately.
by the way, its probably not a good idea to feed him feeder goldfish since they contain lots of diseases from poor care and will likely cause illness in your tank water and causing the feeders to die....
tim
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