There's only one australian lungfish species (Neoceratodus fosteri). It is NOT a good candidate for a 55gal. This species grows slowly but, will reach 67" and 88lbs. As they age they become less limber and will require a tank, at least, twice their length and 1.5 times their length in tank width. That's just to be able to turn around. This is also an expensive species to obtain. The minimum you'll find them for is $450 and then there's the shipping. The average going price is $700 and they retail for as high as $2,400.00. Basically, if you can afford this fish, you should be able to afford to house it properly.