six striped soapfish

i have a six striped soapfish i have owned it for about six years now but it was already full grown when i got it (10 - 12 inches).It is in a 6ft x 2 ft x 2ft tank by itself.It has enough live rock caves to keep it happy but no tankmates. Lately he has been sitting around on the bottom of the tank much more than normal but still has a big appetite.My water params are normal temp. Is also normal 25 celcius i think thats around 70 f.Is he getting old or bored. Also does anyone know the expected lifespan on these fish kept in captivity or even in the wild would be a great help cheers.

 

got any pics? I am not familiar iwht this fish and would like to see it