I would think that if you just dropped it in a part of the sump with virtually no flow that it wouldn't do much good. But surely you can put it in an area with some flow. I just hang a bag in front of the overflow outlet in my sump and it cleans up the water pronto.
I recently read from an Aquarium Frontiers article on carbon usage that a slow but deliberate flow over the carbon is best. Forcing water through it actually decreases its efficiency. The article may have been by Harker or Toonen. Can't remember.
Gentle flow through the carbon is fine. Most sumps have enough water flow that hanging the bag in the middle of the sump will work fine.
There has been some info that high pressure water flow through the carbon that causes it to tumble and disentegrate into fine particles may cause health problems for the tank. People using them in that application (such as a canister filter) have been encouraged by the carbon manufacturers to physically trap the carbon in a way that prevents it from being tumbled around by the water current.
