Short spined urchin

I have a question about these animals. I have a 20 gal tank with 22 lbs
of live rock and 2 short spined urchins came in on the rock. The tank
has been set up since dec. and lately I have been seeing them out and
about all time, when they used to come out only at night. I am wondering
if this could be caused by not enough food in the tank, because I have
changed nothing else.

thanks for any help
Dave