Two of my three FBTs are turning in circle uncontrollably, they have a tendency of rocking their heads back and forth, but when they try to walk or hop they cannot go straight, but rather they can only do so in a circular motion.

All three FBTs are getting up there in years, probably at least 10 years old each. One was affected starting about a year ago, the other more recently. The first one I though had some sort of neurological symptom, maybe a stroke, but now that it is affecting a second frog, I am totally unsure. There other tank mate remains unaffected. All three are still eating, and the sick frogs have no visible deformities.

Does anyone have an idea of what might be wrong?