So what does that mean? Well, carbon use has evolved and devolved in many ways over the past few decades. From in-tank air-driven corner units, to hang-on-the-back flow through filters, to force through canister filters, to mesh bags in a sump--and many other methods. Some of these are better than others. But in most cases either one of two things happens: 1) system water is forced through the carbon at a rate that uses up the carbon’s adsorptive potential to quickly--often referred to as active filtration (you’d have to replace it too often to keep up--and you’d be removing too much “good stuff” from the water) or 2) too much of the carbon is wasted because the system water never penetrates through the entire carbon mass--so some doesn’t even reach its potential before it is discarded--often referred to as passive filtration (expensive waste for most reef carbons). The solution to the first problem is to proportionally reduce the flow through the carbon so that it is saturated over a longer time interval AND to use the correct amount of carbon based on your system size. The solution to the second problem is to force system water evenly throughout the entire mass of carbon so that no dead pockets exist. Our answer to this dilemma has evolved over several years and we call it passive-active carbon filtration. It involves determining the appropriate amount of carbon to use for your system, then placing it in an appropriately sized internal filter, such as the Hagen Fluval Internal Filter* series. The filter should then be hidden from view in the tank or placed in the sump. Make sure you use the appropriate size internal filter to hold the correct amount of carbon for your tank volume. We have found the motors and flow through rates on the Fluval internal filters perfect for evenly using all the carbon surface area over a one week period. Carbon use is NOT a substitute for tank maintenance. It only polishes the water.

The amount of carbon to use for your particular system depends upon variables such as system size, water change regime, biological load (stocking level), feeding techniques and others. Most systems require one half to one cup of carbon per 50 gallons per week. Closer to one cup per 50 gallons for reef systems and as low as one-half cup per 50 gallons for fish-only systems--BUT you will have to experiment a little on your own. A heavily stocked tank will require more carbon than a lightly stocked tank. A system that receives weekly 10% water changes will require less carbon than a system that receives weekly 5% water changes. A system that is fed heavily (3 or more times per day) will require more carbon than a system that is fed lightly (1 time per day). And so on...

Carbon should be rinsed in RO/DI water before it is placed in your system. Any good carbon will contain some black carbon dust that some persons have speculated may not be good for certain system inhabitants. Although this remains to be proven, it is always better to be safe than sorry--and it does sort of make sense that you wouldn’t want carbon dust circulating in your system (would you want to breath it in?). Only use RO/DI water for your carbon rinse--NOT tap water. Activated carbon will pull contaminants out of tap water. Why would you use any of its adsorptive potential before you even get it in your system! Also, place a bit of mesh between the carbon in the filter chamber and the motor unit to prevent the impeller from sucking up carbon pieces--hence clogging or breaking the impeller. The Hagen Fluval Internal Filter* series comes in four sizes for tanks from 25 gallons to 200 gallons. Always step up in size rather than down if you are between two sizes. Some very large tanks may require several units. If so, alternate the carbon replacement between the units on a regular schedule. Well, that’s it. These recommendations apply to freshwater, cichlid and marine systems with few exceptions.

*Unfortunately, since this was written, Hagen has redesigned their internal filters (now they are called the Plus series). They no longer have a one-open-ended chamber that can be filled with loose carbon and topped off with a piece of mesh to protect the impeller. They now open on both ends. A solution here is to use a mesh bag--such as those made in 6x10 and 12x15 sizes by Kent Marine--to contain the carbon. Place the bag solidly in the internal filter chamber, eliminating as many air pockets as possible. Air pockets lead to areas where the water can be drawn around the carbon bag rather than forced through it--and that is our entire point. Also, Aquarium Systems makes the Duetto Internal Filters. These are still one-ended and can be used as the old Hagen Fluval Internal Filter series was. Their only drawback is their size. They are very small and only appropriate for smaller sized tanks.