What is that guy looking for?

One of my three goldfish has a habit of breaking the water surface with its
mouth, like its trying to find more food.  It usually does this right before
or right after feeding time, but sometimes it does it at random times.  Its
not a recent behavior its been doing this as long as I've had the fish.

For the first 3 months or so, this particular fish was initially fed
floating flake food but 6 months ago I switched to sinking foods.  Now its
primary diet is Marine Land's Bio-Blend which sinks like a rock and doesn't
break apart and all three fish swallow it hole.

The other two fish were purchased later so they only ate sinking foods.
Those two usually poke around on the tank bottom, a behavior this one fish
sometimes displays, but not very often.

I'm guessing that its still acting on some learned behavior to look for
floating foods.  Is this a correct assumption?  Or should I be more
concerned?

The fish has no problems swimming and by every other account looks and acts
normal.

Thanks,
E.Otter

Tank stats:
29g
75 degrees Farenheit stablized by tank heater
1 bubble wand and 1 airstone powered by Rena 200
2 Penguin bio-wheel filters: 330 & mini
pH 7.4, Ammonia 0, Nitrites 0, Nitrates 10 ppm.
no plants
Water changes done bi-weekly