Help... Skimmer question!

[fishbait]

Hello, we just picked up a used saltwater system and finally got it all running a few minutes ago. I plugged in the skimmer and it is definitely pumping water, which is good, however it doesn't seem to be "skimming" much. I don't see any froth building up. Is that normal - does it take time, or is there something I need to do to make this happen?

Unfortunately the previous owner threw out nearly all the water from the tank, so we're starting with 90% new water here... not sure if that makes a difference. There is live rock in the system, and I have 3 powerheads running right now as well.

You help/advice is appreciated!