Don't worry about the haters, this forum can be hostile towards newbies. As stated if you take a water sample to your local fish store (LFS) they can probably test it for you. Post the results here.
In the mean time i had a similar thing with my lung, it was a tank mate biting him, once removed he healed up surprisingly quickly.
From your post it seems your tank is far too small for a fish that has the potential to grow to 4'+ you need to make the decision as to whether you are prepared to invest in a 6'+ tank or to try and find someone who can provide him with what he needs.
You say you change the filter, what do you mean by this? You should only ever clean your filter in tank water to remove debris. The bacteria that live in your filter are essential in keeping your tank healthy. Look up the cycling process of a fish tank on google or on here.
Also you should start doing 50% water changes weekly as a minimum.
Any more questions just ask, we were all noobs once despite the attitudes on here!