OK....having kept a couple of lungfish in my time....let's see what I can do for you...
First of all...which lungfish is it? It is unlikely that you're looking at the Queensland Lungfish, Neoceratodus forsteri because trade in that fish is banned under CITES and it is protected heavily in Australia. The African lungfish, Protopterus annectens, P. amphibius, P. aethiopicus, and P. dolloi are occasionally available in the pet trade. The most commonly available is the South American lungfish, Lepidosiren paradoxa.
Regardless of which it is...you need to be aware of a few major limitations in keeping them:
1) All of the lungfish get BIG! N. forsteri reaches 1 m (3+ feet) in length as an adult. P. annectens gets to between 60-80cm (24 - 31 inches). P. amphibius reaches 60cm (24 inches), P. aethiopicus can reach 2m (6 1/2 feet). P. dolloi usually maxes out at about 1 m (3+ feet). L. paradoxa also makes it to slightly over 1m.
2) All of the lungfish have formidible teeth and don't hesitate to use them. Keep children's fingers out of the tank (and your own) or you may suffer some serious damage.
3) It is obvious that lungfish of all flavors need a pretty big tank. A minimun tank length of 90-180cm (3 - 6 feet) is recommended. All lungfish are escape artists so a secure lid is essential but remember that they breathe air so there must be space between the lid and the water's surface.
4) Lungfish are all carnivores. They will eat fish, crustaceans, worms, clams, and other meaty foods. They are not active hunters and most don't need to be fed daily.
5) Lungfish are relatively tolerant of water quality. Standard tropical fish temperatures suit them fine. The best pH range is between 6.0 and 7.0. They are sensitive to nitrate concentration so frequent water changes are necessary.
6) These fish aren't the most personable fish on the planet. They tend to sit still most of the time. Not exactly the fish you want if you like an active tank. Obviously they cannot be kept with other, smaller fish and in many cases they'll get dangerously aggressive even with larger ones. Their formidable jaws mean that they can do significant, even fatal, damage to larger fish. On the other hand, large South American cichlids are likely to annoy the lungfish and may even bite off its filamentous fins.
If you really want to keep a lungfish and have the necessary facilities don't let me talk you out of it. They live a LONG time....up to 30 years....and are fascinating fish!
