If your tropical fish is shaking its body then it is something which you may think is wrong or out of the ordinary and not seen this before. Panic may set in however it is normally not a bad thing, in fact it is quite the opposite (in most cases) and will normally mean it is attempting to breed with another fish in the tank.
There are many breeding rituals you will find fish do and before starting this hobby i had no idea that fish bred the way they do and in fact that there are many different ways they breed. the most likely to shake when breeding are cichlids, more specifically African Cichlids such as those form the rift valley lakes, Malawi, Tanganyika and Victoria.
These are moth brooding cichlids and will shake to attract a mate, it is normally the male that will begin to shake first to attract females to them. They will then spin around in a circle (near the substrate in most cases) and the eggs will be fertalised, this is easier to explain visually so the video below shows some Tropheus Ikola from lake Tanganyika performing this act.
If you don’t suspect it is due to breeding then it would be advisable to look for other signs of abnormal behavior or markings on the fish that you don’t remember and try to do some investigative work based on that regarding what may be wrong with the fish.
Hopefully it will be good news though that your fish are breeding as we believe this is a sign that they are settled in their environment in your tank, so bravo!