Carbon

Activated carbon is very effective at CHEMICALLY cleaning aquarium water

It is great for removing waste products of corals and fish. By itself, it can't usually remove all the waste nutrients in your tank. But it is certainly great at polishing off your water.

Even if you have actually done everything else to to reduce the number of nutrients within your system, carbon can still be effective and helpful in maintaining water control.

CARBON SOURCE

Activated carbon comes in many forms and pore sizes, and there's different considerations to keep in mind, but you should always choose a high-quality carbon. Don't go buy the cheapest carbon, especially one that is not made specifically for aquarium use. Some of these can actually release phosphates back into your aquarium. I think I recall that carbon made from coconuts has this problem. I need to check on this.

PORE SIZE

The size of the pores in the carbon will determine how efficiently it removes waste from your water.
If it has the wrong pore size, it may not be effective whatsoever and you may be totally wasting your money.

HOW TO USE IT

We like to use carbon "passively". This means that we don't put it in a high flow area where lots of water would pass through it quickly.

We put in the low current area. So that the amount of water going through, it is very very slow. So that any changes it causes in the water chemistry also happens very slowly. Corals like stability. You can have a problem if you introduce carbon into a tank that has not had it in a long time. Beware if your water is yellowish and you havent used carbon for some time. If you put a lot of carbon in a very rapidly moving area, you can quickly remove the yellowing compounds but also shock your stony corals. Because what's happening there is a yellow in the water acts as a filter to the lights and your corals have adapted to that particular light wavelength and intensity. By adding the carbon, you remove the yellowing compounds and suddenly the corals are being shocked by lots of extra light. This isnt good.

HOW OFTEN

Because of this, you don't really want to add a large amount of carbon at once. To a system that hasn't had it on before, you will want to start out with us very small amounts. For example for a 29 gallon system, startout with a few tablespoons in a low flow at area and then gradually increase that over time to a regular amount. Typically you should change the bag of carbon every 30 days.

It's really hard to monitor how quickly carbon is used up. Some studies indicate that with carbon left too long in the sump, you can start giving off giving back some of the pollutants that it has taken up so that's why this is important to remember. You don't want to just leave it in there for months and months and months at a time. You can actually be read polluting your system if you do that.