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Do I Need to Change the Charcoal in My Aquarium Filter?

| Updated September 26, 2017

Activated charcoal, also called activated carbon, only lasts for a limited period of time. When it has expired, or absorbed all it can, it ceases to be effective in your aquarium. Theoretically, if you change the water in your aquarium on a weekly basis, charcoal isn’t necessary. But few people do this, and even if you do, using a charcoal filter can add a measure of extra protection.

What Activated Charcoal Does

Charcoal filters out undesirable chemicals and elements in the water of your aquarium. These substances include dissolved organic molecules found in tap water, chlorine and chloramine, some heavy metals, and growth-inhibiting pheromones released by your fish. Carbon does not filter out ammonia, phosphates, sodium chloride, or nitrite or nitrate, although your filter may contain other substances besides carbon that do remove these elements. Carbon increases the level of oxygen in your tank, helps to maintain a proper pH balance, reduces odors and improves your water’s clarity.

When Charcoal Usage May Be Detrimental

A carbon filter can dilute or remove any medication or supplements you use to treat your fish, so you should remove the carbon when you need to add medication to the water. Replacing carbon post-treatment helps remove excess medication. Carbon poses no harm to fish, but it can remove some of the trace elements live plants need to grow, so if you have an abundance of plants, you may want to reduce your carbon usage.

How Often a Charcoal Filter Needs Changing

How often you need to change the carbon in your filter depends on the number of fish in your aquarium relative to its size and the quality of the carbon. The carbon replacement’s packaging usually specifies how long the carbon is expected to last -- in most cases, from one to four weeks, although high-quality carbon may last up to three months. Following the guidelines provided is your best bet unless your tank contains more than the recommended number of fish for its size, in which case you may need to change the charcoal more frequently. If you use filter pads or bagged carbon, you need to change only the carbon; the pad or mesh bag is reusable and only needs changing when it shows signs of wear. A yellowish tint to the water or an unpleasant odor are physical signs that your carbon needs changing.

What to Look for When Choosing Charcoal

Some cheaper carbon products contain phosphates, which can promote the growth of algae in an aquarium; hobbyists who rely on water changes rather than carbon cite often this as a reason for their choice. Ash is another undesirable ingredient. You can reduce the amount of ash and phosphates in carbon by soaking it in water prior to use. Two other factors to consider are the iodine number and molasses number, which indicate the number of micro (small) and macro (large) pores on the surface of the carbon. You want your carbon to contain a balance of micro and macro pores. To achieve this goal, look for an iodine number of around 1,000 and a molasses number of about 225.