On May 20, 12:28 pm, Vreejack wrote:
I have a 29-gal (110-l) heavily planted low-tech tank with 18 fathead
minnows (rosy reds, actually), a couple of Ottociclus affinis, and a
pair of crayfish. Some of the minnows have taken up the habit of
swimming in fast circles along the face of the aquarium glass. Some
of the fishes seem to have their favorite spot and one swims circles
around one of the suckers that holds my filter intake in place. They
usually seem to be synchronized to the music, too, but that is
probably a coincidence. Now even the Ottos are starting to do it, but
only in pairs. The minnows always swim in counter-clockwise circles
as seen from outside the tank but the Ottos do not seem to have a
preference. The crayfish just get dizzy watching them.

Some of the minnows also play a game where they swim towards the
filter return outlet from a safe angle until they get caught by the
stream and flushed halfway across the tank.

Watching them swim like this makes me think of hamsters on a wheel.
Is any of this behavior cause for concern?

There is an internal fungal disease (Ichthyophonus) that will cause
disoriented swimming in circles, however this does not seem to be the
case here. Make sure you have all your electrical properly grounded as
a phantom current in your aquarium can cause excited behavior in
fish.
The usual culprit is a light fixture. A Multi Meter can test for this.

Carl