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Popular Fish Keeping Myths

Common, Generally Unfounded, Possibly Dangerous Myths, Legends, Rumors, and Wives' Tales About Aquarium Care and Fish Husbandry

(The First Tank Guide)

There are many common misconceptions and myths floating around about fish keeping and aquarium care. Some of these are even perpetuated by some pet stores that have not thoroughly looked into the information they are giving out about fish keeping. Here are some of the most common that I run into:

Fish only grow to the size of their tank
This is possibly the most common myth I hear, and I know many stores still tout this as truth. however, it is untrue. To remain healthy and to live a normal and healthy life, fish need to be provided with sufficient space to grow and mature normally. More details on this can be found in the tank population and capacity information here in the First Tank Guide.
You need an air pump and air stones
I'm not sure where this one comes from. The majority of fish tanks do not have any aeration from an air pump, but still many people ask if they have to get an air pump, thinking that the fish will not have any oxygen in the water if there is not an air pump. More information on air pumps and air stones can be found in the Equipment List here in the First Tank Guide.
You need to add salt to keep your fish healthy
This one just strikes me as weird. Fresh water fish should be kept in fresh water - that is water without salt in it. Brackish water fish should be kept in brackish water - that is water with some salt in it. And marine or saltwater fish should be kept in water with enough salt in it to keep them healthy. Adding salt to a freshwater aquarium is unnecessary and possibly dangerous. I believe this one comes from the fact that some diseases can be treated by raising the salinity of the water temporarily to kill off the pathogen (disease organism) causing the illness. However, this requires both a significant elevation in salinity and a change in salinity. Keeping the tank salty all the time will not help with disease resistance in freshwater fish, in fact, it will actually increase diseases susceptibility by keeping the fish somewhat stressed all the time.
Disturbing your gravel is dangerous
OK, this one is partially true. If you never disturb your gravel, then your gravel will develop anaerobic pockets can will likely produce toxic gases. If this happens, then disturbing these anaerobic pockets can have disastrous results for your aquarium as the toxins are released into the tank at large. However, the development of these anaerobic pockets is actually an indication of poor aquarium care in the first place. The gravel should be stirred regularly to ensure that these anaerobic pockets never develop, because if these dangerous anaerobic pockets do develop, they do not necessarily need to have the gravel disturbed to release their toxins. For this reason, among others, vacuuming part of the gravel should be a part of your regular weekly 10-15% water change in every aquarium.
Carbon is absolutely necessary in aquarium filtration
I think the pervasiveness of carbon-based filtration systems has generated this myth. Not only is carbon unnecessary in aquarium filtration, there are cases where the carbon can actually cause problems for the tank. Most aquariums are filtered with filtration systems that use carbon, and these are generally appropriate, however, if you are using liquid plant foods or are medicating the tank, you should assume that if the carbon is working, that the food or medicine is being removed from the tank more or less immediately and is having no beneficial effect. Also, if the carbon in the filter is not changed regularly, it can have dangerous side effects. Additionally, many filtration systems, such as under gravel filters, come with carbon cartridges that are so small that they have no long-term beneficial effects on the tank, and can impede the flow of water through the filter significantly, greatly reducing its overall effectiveness.
Fish require a pH of exactly 7.0 to live
Very little water in nature has a pH of 7.0. Naturally occurring fresh water will have a pH that usually falls somewhere between 5.5 and 8.5, and the pH of the water will fluctuate even at a specific sampling location. Some fish do better in water that is more acid (has a lower pH) while others do better in water that is more basic or alkaline (has a higher pH). However, almost all fish will thrive in water within an acceptable pH range.
Algae eaters and bottom feeders don't contribute to tank population
I think this is just a sales pitch. Any living thing added to the tank that will use oxygen and produce waste will contribute to tank population.
Algae eaters and scavengers are necessary in an aquarium
I think that this is also partially just sales pitch, but it seems that it is also partially pipe dream as well. Many people, particularly new fish keepers want to believe that there is some miracle that they can apply to their tank so that they do not have to take care of it any more. One aspect of this is to assume that the tank requires 'scavenger fish' that will do the cleaning for you. However, this does not seem to have any basis in reality at all...
You can't overpopulate the tank
This one is sort of related to a couple of the previous myths. Unfortunately, it is also untrue. The fish in your aquarium need to be supplied with sufficient oxygen to breath, or they can suffocate in the tank. They also need to have their waste diluted and processed before they poison themselves with it. A fixed amount of water has a fixed capacity to handle these two requirements, and no matter what you do to it, you will still need to meet these needs for tank capacity.
Catfish will keep your tank clean
Actually, most catfish are not scavengers, most are predators (though catfish with little tiny mouths tend to go after little tiny prey - and many catfish have deceptively large mouths). It is highly unlikely that a catfish will do anything to help clean your tank, and it would be unhealthy for them to eat the feces of other fish anyhow. Fish waste should be removed with your regular weekly 10-15% water change, and excess food should be remove promptly so that it does not contribute to poor water quality.
Goldfish are messy fish
Actually, unlike catfish, goldfish are a scavenger. As far as a fish that will go around and clean up your tank for you, these fish are probably your best choice. Of course, this will not reduce the amount of care the tank requires, because the tank will still need all the same maintenance and care that the tank needs, including the regular weekly 10-15% water changes to keep the system clean and healthy.
You can't do water changes while the tank is cycling
Actually, not only is this untrue, but it is detrimental. You can and should do water changes while the tank is cycling. In addition to possibly speeding up the cycle, provided the water changes are small, it will actually reduce the stress your fish go through from "new tank syndrome" and can greatly reduce your headaches and losses while the tank is cycling. More on water changes in the cycling aquarium.
Algae eaters will keep your tank clean
I find this one extremely frustrating. I know that in part, this is also part sales pitch and part pipe dream. Many aquarium owners want some kind of magic wand that will do all their work for them, and it just doesn't exist. The only reliable way to remove algae from the tank is by scrubbing, a fish is unlikely to do so. Check out my algae control tips for more information.
Your tank needs to sit before you add fish
A lot of people will tell you that your tank has to sit for a week, a month, three weeks, two months, or some other arbitrary period of time before you add fish. Sometimes this is referred to as "cycling" the tank, though the tank can't actually start to cycle until you have fish. Other times this is just referred to as 'setting' the tank. Either way it is an unnecessary part of setting up your aquarium.
Cycling with fish is dangerous and cruel
Actually, cycling your new fish tank with fish is the fastest, easiest, safest, most reliable, and most humane way to cycle the tank. Cycling your aquarium is only risky to your fish if you cycle with too many fish or don't keep up with the necessary extra water changes while the fish tank is cycling. On the other hand, cycling without fish, also known as fishless cycling has many risks and is complicated and expensive. In fact, if you do a fishless cycle you are more likely to then add fish to an uncycled tank, and find yourself unknowingly cycling the tank with too many fish and without enough water changes.
Tiny, like 2, 3, or 5 gallon fish tanks make good beginner tanks
Actually, tiny fish tanks like these are very high maintenance and prone to problems and failures. These tiny aquarium can only be recommended to very advanced aquarium keepers who have a lot of experience maintaining fish tanks and who are ready and willing to take on the challenge one of these tiny aquarium presents - and is not concerned about the cruelty inherent in such tiny tanks. Good beginner fish tanks are always in the 15-30 gallon (that's about 60-120 liter) range.
Adding water from an established aquarium will make your fish tank cycle faster
The beneficial bacteria of the biological filter in your aquarium grow almost exclusively on the surfaces of solid objects in the fish tank. Moving water from an established fish tank to a new fish tank will move some bacteria from the older tank to the newer tank, but this difference will be very minimal, and will have little to no effect on the time required to cycle the new aquarium.
There are a bunch of chemicals you need to keep adding to your tank to keep it healthy
There are a bunch of chemicals available on the market that you can add to your fish tank. However, with very few exceptions, the only thing you need to add is a dechlorinator, and you only need to add that to new water being added to the fish tank (preferably before the water is added to the tank).
Feeder Goldfish will Cycle Your Fish Tank Faster
This is related to the misconception listed above that goldfish are messy fish, coupled with the misconception listed next that a fish tank will cycle faster with a higher ammonia level. Goldfish will not cycle your tank any faster, and feeder goldfish are a bad choice for cycling your aquarium.
An Aquarium Needs a High Ammonia Level to Cycle
Actually, ammonia displaces dissolved oxygen from the water, so the higher the ammonia level in the water, the longer it will take the biological filter to cycle. This is among the many reasons that it is so important to do small water changes while the fish tank is cycling.

Each of these myths is, to put it quite bluntly, incorrect. Some of these are even dangerous - either to you or to your fish. Some will just make fish keeping more expensive for you, or maybe just more frustrating. Remember, look into the resources you are using, and evaluate that information they are providing - then double check that information against the First Tank Guide to see if the information you have gotten still holds water.

"Thanks again. It is such a "good thing" when there are people like you willing to take the time to help people like me."
August 26, 2003
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"I want to thank you for your time to answer my incessent questions. I have talked with you several times and have always taken your advice and learned from it. I love your website and visit it often. I enjoy my aquarium more than I ever thought I would, therefore I want to learn everything I can about how to maintain my tank to keep it healthy and beautiful. I apologize for taking up your time with my questions and I appreciate the wonderful information you have shared with me. Thank you again."
October 22, 2006
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