If I'm understanding you correctly your keeping a 20 year old Lungfish in a 20-50 gallon tank and are unsure of the filtration and whether it's suitable for the size tank never mind the fish you have in it. The first thing I have to say is unfortunately you have a lot of research to do as it doesn't appear as though you have even the basics of fishkeeping down, as such it's going to take a lot more then the Lung stickies to help you. At 20 years old I'd imagine most lungs would be easily 2-3 feet long so the fact that the lung still has room in the tank that you think is between 20-50 gallons(I'm assuming) suggests that he's horribly stunted. As a 20 gallon long is 30 x 12 x 12 and a 55 is 48 x 12 x 21 which means the lung should literally be taking up the entire bottom of the tank at his/her age. Just because an animal has lived in certain conditions doesn't mean it has been thriving, many animals can live in atrocious conditions and Lungs are one of the strongest survivors alongside goldfish in the aquatic kingdom.
As for the marks/injuries they look like in my opinion to be fin rot or ammonia burns due to his/her living conditions catching up with him as while Lungs are survivors eventually the living conditions will eventually break down their resistance and diseases will take hold. The first thing to do is buy a test kit since water quality is paramount and a kit is considered standard fare for anyone keeping fish any petstore that sells fish will almost certainly have them. The cheapest test kit is the API strip test kit which costs about $3.99 but your better off getting a API Freshwater Master Test Kit which is about $16, what ever test kit you get should have a test for PH, Ammonia, Nitate, Nitrite and Carbonate Hardness. Once you know your water parameters we should be able to help you.
