Maintaining good water quality and monitoring the filter cartridge in your fish tank is the most important aspect of keeping aquarium fish of any species, as the fish will quickly become sick and die if the water is dirty and polluted with ammonia, nitrite, and nitrates.
To keep the water safe and clean, you should run an efficient aquarium filter that’s the correct size for your tank. Although all fish tank filter systems should work quietly in the background without detracting from the enjoyment of your fish, they do require regular proper filter maintenance.
But how often should you change the filter cartridge in your fish tank?
The frequency with which you change your filter media depends on several factors. However, as a general rule of thumb, you should replace the filter cartridge once a month or so or in line with the manufacturer’s guidelines.
Read this guide to learn more about maintaining your fish tank’s filter system and how often you should replace the different filter media.
How Often Should I Change My Fish Tank Filter Cartridge?
Some fish tank filter units use an all-in-one filter cartridge containing a combination of mechanical, biological, and chemical filter media. These can be a good choice for beginners to the hobby since it takes away the need to choose what individual filter media you use in your tank and is very convenient.
The main drawback to using this filter type is that replacement cartridges tend to be rather pricey.
The frequency at which your aquarium filter cartridge should be replaced depends on several factors, including the tank size, the species and number of fish you keep, the type of filter you use, and the general water quality in your aquarium.
However, most filter cartridge manufacturers recommend replacing the filter once a month.
Should You Routinely Replace the Filter Cartridge?
Usually, the product manufacturer will provide fairly unbiased guidelines on the packaging regarding replacing the cartridge.
However, filter cartridges are quite expensive to buy, so you don’t want to waste your hard-earned cash by replacing a cartridge when you don’t need to.
When I ran a filter system that used cartridges, I would rinse the cartridge in dirty tank water once a fortnight as part of my partial water change routine. This got rid of any sludge or clogging that could otherwise have prevented water from flowing freely through the cartridge and effectively extended the life of the filter cartridge by a couple of weeks or even more.
What About Chemical Filter Media?
That said, the filter system I used didn’t contain a chemical filtration element. Chemical filtration is designed to remove heavy metals and some other contaminants from the water, but not all cartridges contain them.
If the filter cartridge in your fish tank contains chemical media, usually activated carbon, you might need to routinely replace the cartridge once a month, even if the other elements are still working fine.
That’s because activated carbon binds with the compounds it removes from the water, eventually becoming saturated with contaminants, so it’s less effective. Usually, that takes around a month, which is why most manufacturers recommend replacing cartridges containing a chemical filter element on a monthly basis.
You can sometimes customize your filter media depending on the filter type you’re using. This would enable you to replace the chemical media while leaving the biological media in place for another week or more if it doesn’t require replacement.
What Not To Do!
Never be tempted to try to prolong the life of the filter cartridge in your fish tank by washing it in hot water under the kitchen tap!
Tap water contains chemicals, including chloramine and chlorine, which are extremely dangerous to your fish and will kill all the beneficial bacteria you’re trying to cultivate in the filter media.
In addition, for the same reasons, do not use dish soap or any other form of household cleaner to freshen up the filter media or clean the filter box.
When Should You Change the Filter Cartridge?
The filter cartridge in your fish tank typically comprises mechanical, biological, and chemical media.
As mentioned above, by rinsing the cartridge periodically, you can remove dirt that prevents the mechanical element of the filter system from working properly. This allows water to pass unhindered through the biological filter media so that the beneficial bacteria colonies that live there can process any ammonia and nitrates in the water, rendering it safe for your fish.
I recommend that you inspect the filter cartridge and routinely clean the filter unit box and impeller at least once a month. However, you might need to do that more frequently if you keep very dirty fish, such as goldfish or larger carnivorous species that generate a lot of waste.
Take a close look at the filter cartridge in your fish tank. If the fibrous filter media appears frayed, thinning, or very heavily stained, it’s probably time to replace the cartridge.
However, there are some other signs to watch out for that indicate when your filter cartridge has become ineffective and needs replacing.
Deteriorating Water Conditions
I recommend that you test your aquarium water at least once a week to ensure that levels of ammonia and nitrites are zero and that nitrate levels are around 20 ppm or less.
If the nitrate levels are creeping up, that’s a sure sign your filter cartridge needs urgent replacement.
The presence of ammonia or nitrites in the water means that you must change the cartridge immediately and carry out a partial water change urgently to bring those levels down to zero. Nitrates can be around 20 ppm or less, but if the levels get too high, you risk poisoning your fish.
Fluctuating water parameters are sure to stress your fish, potentially leading to a failure to thrive and even mass fish kills if you keep sensitive species. Hence, it’s crucial that you ensure the water chemistry in your aquarium remains as stable as possible.
The Water Smells Terrible!
If your fish tank starts to stink, it usually means something has gone seriously wrong with the filtration process. In that case, it’s most likely time you change the filter cartridge in your fish tank.
If the filter cartridge is clogged, water can’t flow through it across the biological media, so the beneficial bacteria that live there can’t process the ammonia the dirty water contains. When that happens, the water quality suffers, and the tank starts to smell stale and unpleasant.
Of course, the pong could be caused by the presence of a dead fish or shrimp in the tank. On more than one occasion, when my fish tank started stinking, I checked my filter and found it to be working fine. On closer inspection, I discovered a dead, decomposing tetra lurking on the bottom of the tank, hidden among the plant bases.
I removed the deceased fish, carried out a partial water change, and vacuumed the substrate thoroughly. Within an hour, the tank smelled as fresh as a fish tank ever does, and the problem was solved.
After that, I carried out a head count every day at morning feeding time to make sure all the fish were present and correct. That way, if any were deceased, I could remove the fish before it began decomposing and contaminating the water.
Although an algal bloom can cause cloudy water, your tank might also appear murky if your filter cartridge needs changing because the mechanical filter is clogged, and the water can’t flow through it.
Suppose you don’t have a replacement cartridge on hand. In that case, the problem can be temporarily solved by rinsing the cartridge in tank water to remove the material clogging it, as explained above.
Poor Water Flow
If you run a filtration system that usually pumps out a pretty decent current around the tank and notice that the water surface is barely moving, that could indicate that your filter cartridge needs replacement.
A clogged filter cartridge usually causes the problem, although it could also be that the filter unit itself is faulty or needs a thorough cleaning. If the filter cartridge is spent, simply change it.
However, the filter pump’s impeller is sometimes blocked with sludge or bits of plant material and can’t spin freely, which immediately prevents the water from flowing through the cartridge.
That problem is typically quick and easy to fix. All you need to do is disassemble the filter box and pump housing, then gently use a filter brush to remove dirt and debris from the impeller. Give the filter housing a good clean while you’re at it to remove dirt and algae growth deposits.
When you reassemble the unit, install a replacement filter cartridge in your fish tank, and switch it back on, the water movement should return to its normal strength immediately.
The biological element of the filter system contains beneficial bacteria colonies that process harmful pollutants in the water, such as ammonia and nitrates. Of course, if you replace the filter cartridge, you lose those bacteria, leaving your tank vulnerable to an ammonia spike that could kill your fish.
But you will need to replace that filter cartridge in your fish tank on a regular basis, so how can you preserve the all-important bacteria in the old cartridge?
Retain the Old Cartridge
The most obvious thing to do is use the old cartridge for a month or so to assist the old one, effectively running two filters simultaneously.
Of course, to do this, you’ll need to have a spare filter unit that you can run alongside the existing one and enough space in your tank to accommodate that, which might not be a practical solution.
You should remove the old filter after around a month and then install the new one outright. That ensures the aquarium’s balance is maintained by allowing the new cartridge sufficient time for beneficial Nitrosomonas and Nitrobacter bacteria to colonize the new filter medium.
If you decide to try this option, you’ll need to ensure that the flow rates remain correct for the fish species you keep. If the flow rate is too fast, your fish could become stressed, especially if you have small fish or weak swimmers, such as fancy goldfish.
Colonize the New Filter
If you can’t run two filters simultaneously or the old one won’t fit into the filter unit with the new one, you can use the old, dirty filter to populate the new one with bacteria.
Start by soaking the new filter media cartridge in the fish tank for an hour or so. Now, take the old filter out of the filter unit and press it gently against the new cartridge to hold the biological media against the new surface.
That process transfers some of the bacteria from the old media onto the new.
Although it might not seem like much bacteria has been transferred over onto the new filter media, don’t worry! Remember that every surface in your aquarium, including the substrate, provides a surface on which the bacteria is already growing and helping the filter unit.
Use Filter Booster Products
It does no harm to use a good filter boost product when you change your filter cartridge.
Filter boosters are liquids containing concentrated beneficial bacteria that you add to your tank water.
These products are commonly used to help kickstart the nitrogen cycle in new tanks, although you can use them to support a new filter cartridge, too.
I hope you enjoyed our guide on how often you should change the filter cartridge in your fish tank. If you found the information in the article helpful, please take a moment to hit the share button before you go!
Most manufacturers recommend replacing the filter cartridge every month to keep the system running efficiently. Sometimes, you can extend the life of the cartridge by rinsing it in tank water to remove sludge and debris that would prevent water from flowing freely over the filter media.
What tips do you have for prolonging the life of your filter cartridges? Share them with other readers in the comments box below!