Shy cories

Our cories are really shy. They were more sociable (although still
fairly skittish) when we first got them. As far as the water goes, it's
a steady 78 degrees F, 0 ammonia and nitrite, and very little nitrate
(we keep it as low as possible, between 5 and 15). As far as decoration
and cover, there are no live plants, two caves (one larger and one
smaller), and two plastic plants. The tank itself is 20 gallons, and we
use hard (about 17dH), buffered (about 7dH), dechlorinated (chlorine
only -- no chloramines), basic (pH about 8.2) tapwater. There is a
bubble wand along the back, and they used to play in the bubbles, but
they haven't lately very much. The particular species is C.
melanistius.

Any ideas on why they're hiding out? I suspect that they might want
more hiding places. Another possibility is that they feel nervous with
no other fish in the tank, but I don't know why they would wait until
now to express that. One of our land-dwelling cats is mildly interested
in the tank -- sometimes he will stand up with his front paws on the
stand and look into the tank. Might he be stressing them out? We are
feeding them one small Hikari algae wafer or two or three Omega shrimp
pellets once a day in the evening. There are six of them. Should we
feed different foods, more food, more servings, or do anything else
differently?

It's kind of frustrating -- I never have seen them much, and I've barely
seen them at all lately. They are so cute when they play and dart to
the surface.

Thanks for any ideas.

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