So You Didn’t Cycle Your Aquarium Before Adding Fish

All too often the question comes up online- why are my new fish dieing? The first answer is usually another question- did you cycle your tank? Nearly every time the answer is no.


The aquarium cycling process is a very important step that’s skipped far too often by beginners. This usually leads to dead fish and a frustrated aquarist. But fear not! Even if you didn’t cycle your tank before adding fish all hope is not lost.


First, a quick overview on what this cycle is and why it’s important. Basically, all of your fish are continually releasing waste into the water, most importantly in the form of ammonia. Ammonia is deadly to your fish in even small quantities. Luckily, there are bacteria that love ammonia and want only to consume it. This leads to another problem, though, the bacteria’s own waste product- nitrite. Nitrite is also quite harmful to fish. But, just as there is a bacteria that loves ammonia, so too is there one that loves nitrite. These bacteria then output nitrate which is relatively safe for most fish. These bacteria are very easy to grow, but, it takes time. Normally you would cycle a new tank sans fish, but, if the fish are already present then you’ll have to approach things a little differently.


In an uncycled aquarium containing fish a rise in ammonia is inevitable. However, this doesn’t necessarily mean the ammonia level will spike to deadly levels now or ever for that matter. Lots of factors come into play, the most important being the amount of waste the fish are producing and the volume of water they are in. A few tiny fish in a very large tank will likely be just fine even if the tank was not cycled beforehand. Nonetheless there’s no better way to know whether or not ammonia is building up to dangerous levels than with a good test kit. If you didn’t cycle your tank before adding fish then acquiring the proper test kits to monitor the water quality should be your first goal.


Should you find the ammonia level to be unsuitably high (above 1ppm) or the fish are showing any signs of ammonia poisoning (red gills, gasping for air, discoloration, sluggishness) then your next goal should be a water change to bring the level down. Big water changes usually aren’t recommended since they can be stressful to your fish in their own right, but if they’re swimming around in ammonia rich water it’s a necessary risk. Change around 50% of the water, being very careful to match the pH and temperature, and retest for ammonia. 50% should be enough to bring the ammonia level down into a safe range but if it’s still too high you’ll need to do another water change. Lots of aeration can also help drive some of the excess ammonia out of the water so add an air pump if you don’t have one already.


Once the ammonia level is in check it needs to be kept there. In essence this means reducing feedings until the cycle is complete. Since fish food is ultimately the source of the ammonia, cutting the amount of food the fish are fed will cut the amount of ammonia they produce. Most fish will get along just fine on being fed only once every other day or so. Still, make sure to monitor the fish carefully for underfeeding. The clearest sign is a sunken belly. Continue to monitor the ammonia level during this time as well and do more water changes if it creeps back up. The cycling process will likely take a bit longer than it normally would without fish since the ammonia level is kept so low, but it will finish eventually. Bumping the temperature up a bit can help the cycle progress a little faster. The beneficial bacteria actually do best up in the mid 80s, but this is far too warm for fish. Still, you can increase the water temperature up to around 80 to speed things up just a bit.


Adding fish to an uncycled aquarium is never ideal, but with proper knowledge and action it doesn’t have to be a death sentence. Of course simply returning the fish or moving them to another tank that’s already cycled is another option. But with proper monitoring and care a tank can be cycled with fish and without loses.

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