Would my floss bag be the best place for the carbon if it were in a sock or something?

Carbon works by adsorbtion.  AC is very hydrophopic (that means it repels water).  Almost all the hydrophopic molecules you might find in your reef tank water are ogranics.  Essentially the same stuff that your skimmer removes.  Any molecule that floats by AC and is itself hydrophopic will adhere to the carbon surface (hence adsorbtion).  Once attached it is very unlikely to release.  It might release if the carbon suface was completely filled with hydrophopic molecules and a molecule even more hydrophopic were to come by.  That really hydrophopic molecule might displace one of the weaker ones.  Backflushing and watching DP is only a concern if you have particulate matter that physically plugs the spaces between carbon pieces.  If this is a problem, a solids (mechanical) filter in line upstream of the carbon would help to fix it.

Oh Carl, I think you mean that AC removes chlorine.  Chlorine is not chloride.

ClO- + C = Cl- + C-O
hypochloride + activated carbon = chloride + oxidized carbon

Carbon does not have a useable life per say. It really depends on the levels of the contaminants in the wastestream. Once the AC media is impacted (it cannot adsorb or absorb anymore) it is no longer useful. When that is... who knows. I would look at the flow through the media and perform a calculation of how many times your total water volume would have passed through. The more total passes, the better.

Personally, I don't run AC. I guess if I did I would run it a week (depending on the size of the media bed I use) and flush it out really well. If you want to just leave it in the system, it won't leach or permeate anything bad except what was already in your system. You could make it a habit to flush it out really well every couple of weeks to a month with a water change. Most of what you will remove will be organics or particulates. I would say that after 3 or so uses it is spent.

Of course one could place it into a vessel and monitor the fluid pressure through. When the fluid pressure gets to X-psi, it's time to backflush. If backflushing fails to lower the pressure, it's time to replace the media. But, that sure does sound like alot of work!

As a side note, did you know that AC removes chlorides  . And you wondered why your SG dropped when you ran AC! Honestly, it absorbs a trace amount... don't worry.

Mark Peterson wrote: Alternatively, I would use carbon. Place a fist sized bag in an area of good water flow, like hang it around a pump inlet. Carbon will remove chemicals from the water that you cannot easily test for and will also remove a few that skimming cannot.

Just wanted to add a bit of explanation to ensure that people use it properly. Activated carbon is a variety of orcanics that have been burnt. It could be husks, crushed bone, crushed coconut shell, whatever. The burning process causes it to have many fractures, cracks and voids.

Water flows through the filter BED of AC. contaminants are then ADsorbed or ABsorbed by the AC. Generally, carbon utilizes ADsorbtion which basically means that particulates of contaminants get caught by the media (AC).

A couple of important points: 1.) just sticking a bag of AC media into a volume of water provides very little benefit. Water must flow through the media for it to be effective. 2.) AC can be "Backflushed" which means that the water flow can be reversed flushing the ADsorbed contaminants off of or out of the media and back into the wastestream. Be cautious when removing or placing the AC media in systems since one can easily send a concentration of contaminants back into the system.

Different "formulas" of AC are available to target different varieties of contaminants. Most (too general?) are designed to remove suspended particulate and light metals.

I guess I am a filtration