how to tell if underfeeding goldfish?

I have a shubunkin, a common goldfish, and two fantails in a 29 G tank.
The fantails are much smaller than the single tails. I feed them twice a
day, flake food from Tetra in the morning, pellets in the evening. I
also toss in a spirulina disc at night. I'm wondering if the fantails
are not getting enough food. My fantails are shaped differently from the
fantails I see at the pet stores - my fantails are a bit more elongated
and not quite as rotund in shape. They are still quite fat if one gets a
view from the back, though. Poop looks normal. Are they being underfed?

Incidentally, the common goldfish, which is the largest, is a bit of a
food bully but it and the shubunkin usually bully each other leaving the
fantails alone to chase after food scraps. When I worry that the
smallest fantail has not been getting enough food I put some food
pellets in a small glass saltshaker without the lid, the mouth of which
is only large enough for the fantail.

y-m