Skin Irritation

Actually, my fish are not cichlids, they are local wild fish.

I've got two tanks that are fully cycled with ammonia and nitrites safely
under control. Nitrates are also within acceptable range. Besides the water
being a little hard and some variations on alkalinity, everything is pretty
well within range.

But while my fish look great, and behave and eat normally, I still see them
occasionally flashing against decorations and sometimes doing some sort of
shaking motion.

I was just wondering if there is something else wrong with the water, or
perhaps it is normal for this to happen from time to time with fish that are
kept in an aquarium.

Jonathan