Hey, I just saw this thread. If fish in and of themselves aren't a problem for pacman frogs, a silverside shouldn't hurt them. They are commonly fed to garters which are very sensitive to salt and chemicals (I keep garters, so I'm very familiar with silversides). If you buy them from a petstore supplier like Petsmart or something, then they should have no salt or parasites, even though they are marine fish.
Silversides are really common 'safe' feeders for a lot of reptiles if live fish aren't available (basically, if it can eat fish, silversides shouldn't hurt it). I personally wouldn't feed a lot of them to a pac, just because I wouldn't feed them mostly fish to begin with.
Just thought I'd post since it doesn't seem like anyone here uses silversides

I do agree with Grif though, if you aren't sure it's better to ask first and then feed.