Running carbon all the time in marine is best, while using only relatively-small amounts at a time and changing it weekly. I invite other opinions on this, of course. With cichlids, the current trend is no-carbon, interestingly enough.
JLM, I can't remember the brandname of my airstone protein skimmer. It came in plastic shrink-wrap and cardboard backing, not a box. It consisted of an arycrilic tube, about 2"in diameter, about 7" tall, with a collection cup that fit ontop with a lid, and a bottom had an attachment that allowed for a wood aireator. It was rated at up to 30g I think.
But those ratings are always egaggerated, not to mention, I have a 55g tank. My modifications were this:
I took a plastic cyndrical tube from a gravel-siphon-vacummer, of the same diameter but of about 10" in hieght, and super-glued the two tubes together on each end to make one long 17" tube.
The collection cup remains the same, but small vinyl tubing that I added allows for this cup's drainage into a plastic milk jug.
I drilled a hole about 4" or so underneath the collection cup, a little smaller in diameter than vinyl air tubing. This is where my addition of water off the main pump's 1/2" PVC water line comes from, regulated by a PVC ball valve. The airline tubing is what connects the valve to the protein skimmer, but used for water instead of air.
The airline and wood airstone underneath is still the same, powered by a good aireator.
The unit is kept in my sump, not the tank, or in-line. My sumps are over 17" deep with water, so the skimmer can fit nicely inside the system. Water exits out the bottom of the 17" tube, and the gunk is left in the collection cup at the top, or in the plastic milk jug that's attached.
All this makes it a good counter-current airstone protein skimmer, far surpasing my expectations for my 55g. It removes any and all gunk!
I have to say, I'm proud of myself with that one. Didn't cost me more than $40 CDN.