Possibly heavy metal poisoning. The description is classic mercury poisoning symptoms. It takes time for it to build up in a snake before it gets to a level considered toxic and it effects the nervous system. This is least likely if you're using Sally's or Ocean Nutrition brand silversides because they come from the arctic. Closer you get to the poles, the less heavy metal contaminants fish have in their bodies. But not all "silversides" are actually silversides.(the good ones are not a true smelt, they are arctic "pond smelt") "silversides" caught off the coast of California for example, are true smelt and are too salty, contaminated with mercury and PCB's and to top it off, they have moderate levels of thiaminaise. They aren't going to hurt your snake if used in moderation and part of a varied and nutritious diet but used nearly exclusively over long periods of time, mercury or thiamnaise poisoning can occur.

I would stop feeding them that fish immediately and throw it out regardless. These symptoms can nearly always be traced to the fish. I've lost roughly half the snakes that developed symptoms like this but immediately switching them to pinkies (scented with fish if necessary) with supplements has reversed the condition in a month or so for some snakes and they lived. Good luck.