lungfish growth

Oh yes it is normal If you don't like it, feed him less

Let me rephrase my comment...

It is common / normal if given a lot of food, but in reality, and in the wild, it is highly unlikely that they do grow this quickly as food is on a more "limited" basis per se.

When fed daily they do grow this fast, but when on a more "natural" feeding schedule of several times a week, they do grow noticeably slower.

That seems to be the case. I've a marble lungfish acquaired in April 2006 at about 7". Now measured at 19". It actually grows 12" in 11 month period.
Feeding in variety of pellets, twice daily.