Mike is correct. However I use plastic media that has a large surface area in my packed column, and although I presently use a 10 inch 5 foot section of PVC that the water falls through at the top, I'm going to switch to 5 five gallon buckets in a row loaded with the packed material and a plastic screen in the bottom of each to keep the plastic material inside. This way I connect the buckets w/ a 2 inch space between each which allows more hydrogen sulfide, C02, and nitrogen gas to escape. It will also do a better job of oxygenating the water since it has more access to the air.
You do know that well water is virtually devoid of oxygen when it first comes out of the ground and is saturated or even supersaturated with nitrogen gas? You need to blow off all the undesirable gases and add oxygen which the "packed column" does. If you have too much nitrogen gas from your ground water supply your fish can actually develop the "bends."
Do you have iron in your ground water? Larger fish can tolerate it, but it depends on how ppms you have. It will also discolor your water which is not aesthetically pleasing. I am presently making a prototype of a double sand/gravel filter to collect the precipitated iron after the water goes through the packed column. I won't know how well it works until next spring as there is no need to run my well until then in my trout pond. Pumping well water is expensive and I only do it from late spring to fall.
If you have anymore questions email me. I've made some mistakes so I've learned alot!
I also have pics of recently harvested browns, brooks, and rainbows on my website at
http://www.ligtel.com/~jjbaird/bairdfish2.htm