Top Freshwater Aquarium Myths

Myths and legends abound in the aquarium hobby. Like the telephone game, anecdotes get passed along, and with each telling, some bits are removed, some changed, and some added. In the end, we have a mash-up of good info and bad info. I know that in my own adventures in babysitting fishkeeping, I’ve been swayed by false information and, unfortunately, passed it on to others. So with this article, I would like to address some common misconceptions in freshwater fishkeeping. Below I have broken up several myths into a few topics.

Cycling and Bioload

“Allow a tank to sit for (some long time) prior to adding fish.”

This is one of those “some good, some bad” tips, and here’s why. It’s a good idea to set up a new tank and let it run, fishless, for 24 to 48 hours. I say this because it allows time for a few important things to occur. During this time, you can verify that there are no leaks, your stand is sturdy enough to support the weight of a filled tank, and that all your equipment is working properly. It’s easier to deal with a problem without the added variable of live animals. Secondly, if you are using a heater (which you should, more below) it can take several hours for the temp to stabilize. This is especially true if you are going old school and using one of those manually adjusted heaters. Beyond all that, there really is no benefit to letting a tank “settle” for days or weeks prior to adding fish or other pets.

A lot of times, people I talk to will say something like, “my tank is set up and has been sitting for two weeks. So it’s cycled and ready for fish.” Unfortunately, that’s not how cycling works. If you read my article on the Nitrogen Cycle, you’d know that cycling a tank is the process of establishing a healthy colony of beneficial bacteria in your tank and filter. In order for this to happen, the bacteria need to consume ammonia. In an aquarium, this ammonia primarily comes from fish waste. Without a source of ammonia, the beneficial bacteria cannot be established, therefore preventing your tank from cycling. In short, your tank that has been sitting there for two weeks without fish is just as sterile as on day one. In fact, it may be more sterile, because any stray bacteria that came in when you first filled the tank have long since starved to death.

The only exception to this is if you are using some bioactive substrate like potting soil or Eco-Complete. These come with their own culture of bacteria and will produce plenty of ammonia as it begins to breakdown in the water. There are also some other techniques to cycle a tank without fish, read my article on the Nitrogen Cycle to learn more.

“You can speed up cycling a tank by using old tank water.”

The beneficial bacteria (BB) that we encourage to grow in our aquarium filters live and grow on wet surfaces. These surfaces are found in abundance within the filter. BB will also happily grow on aquarium walls, decorations and plants, and in the gravel. While there may be a few stray bacteria that find their way into the water column, there generally are not enough to make a big difference for speeding up a cycling tank. So, unfortunately, there is no noticeable benefit from using old tank water to jump start a cycle. The only benefit you might get is if the old tank water is dirty and contains ammonia. This ammonia would then nourish the bacteria you are starting to grow in your new tank. Hopefully, water from your old tank doesn’t have any measurable ammonia, because that would mean there is something wrong in your old tank.

You can however, speed up the cycling process by adding used gravel, decorations, or filter media to your new tank. These used items should be fresh from the source and still wet, because BB die quickly when exposed to air.

There are products out there that claim to speed up the cycling process, but more on those below.

“Cleaner fish and snails don’t add to the bioload of a tank.”

Fish are fish. They all need food, they all produce waste, and they all die. The same goes for snails. Waste produced by these animals will eventually break down and leech ammonia and other compounds into the water. Sure, “cleaner fish” may make the tank look nicer by eating algae, but they are no different from any other pets in the aquarium.

You might be thinking you’ve found a loophole by not feeding those cleaner pets any additional food. That may work for a while, but those animals are still digesting the algae they are eating and producing waste. Think of it this way: algae may not look pretty, but it isn’t toxic (usually). A snail comes along and eats some of that algae, producing waste. That waste gives off ammonia. Which is better to have in the tank, ugly algae or toxic ammonia?

Eventually, cleaners may die in the tank and go unnoticed for some period of time. Those decomposing corpses will pour ammonia and other waste products back into the water column. This is particularly attributed to snails. If you have a large colony of snails in your tank, and they all die at once from some chemical or lack of food, you can experience a harmful ammonia spike.

The best way to manage waste is to control feeding, clean your tank regularly, and do appropriate water changes. Plants will also help manage waste products, but don’t rely on them to do all the work. Check out my article on Basic Aquarium Maintenance for more info on maintaining a regular cleaning schedule.

“You need Instant Cycle Product X to start a new tank!”

There are many products out there claiming to instantly cycle your tank. The majority of these are, to use the technical term, snake oil! At best, these products contain compounds to help encourage the growth of beneficial bacteria (BB).

As far as I know, the only product that comes close to “instantly cycling” a tank is TurboStart by Fritz Zyme. It is actual, living bacteria. It stands out from the rest, because it must be kept refrigerated and has a short shelf life. Other products can be found stored on regular, unrefrigerated shelves with suspiciously long expiration dates. TurboStart needs to be used at the same time as adding fish to a new aquarium. This is because that bottled bacteria culture needs a source of ammonia to grow and seed your filter media. If you put it in, and wait days or weeks to add fish, you might as well have not done anything at all. This is because your BB will have starved to death in that time.

On the Fritz Zyme website, there are links to independent studies, demonstrating this product’s effectiveness vs. other brands.

Personally, I feel the best way to cycle a tank is the old fashioned way, with fish, close observation, and patience.

Cleaning and Maintenance

“Why is my fish sick? I change the water and wash the gravel regularly.”

I’m always surprised to hear that some people think their aquarium gravel should be removed, washed, and returned to the tank as part of normal aquarium maintenance. Sometimes, those folks even mention using dish soap to clean the gravel.

Lord, give me strength!

Honestly, I don’t hear this one often, but it’s usually from older folks (no hate). I guess this is a throwback to a time when we didn’t really consider how certain bacteria can actually benefit our aquariums… or when under-gravel filters were all the rage.

Beyond simple gravel vacuuming, you shouldn’t clean your aquarium gravel. The gravel bed is an important location for the growth of beneficial bacteria. If you routinely clean it with soap or some other product, you are killing that precious colony of bacteria, each and every time. Think of the gravel bed as secondary filter media.

You can clean old gravel if you are planning to start a new tank, and it’s been sitting in your garage for months, sure. I strongly recommend you don’t use any soap or harsh chemicals. That stuff can leave a harmful residue in the gravel. If you insist on cleaning gravel, simply rinse it with dechlorinated water or a weak hydrogen peroxide or white vinegar solution.

“My water looks clear, so I’m good, right?”

Wrong! Well, maybe. Just because your water “looks clean” doesn’t mean it’s good for your fish. Ammonia, nitrite, and nitrate don’t give your water any funky colors, yet all of these compounds are toxic to your pets at differing concentrations.

To use an over-the-top example to illustrate this point: distilled white vinegar looks just like clear water, but you wouldn’t fill a tank with it, would you?

The only way to be sure your water is “good” is to check your parameters. So do it, whether your water looks clean or not. Not sure what water parameters are or how to test them? Check out my article on Aquarium Water Parameters.

“My fish don’t look sick, so everything must be fine.”

This one is related to the one above. It’s true that sick fish will usually show signs of distress. This is the primary way that we aquarists know something’s up. The other way is by doing regular water tests.

Fish will not always let you know when they’re in trouble. Here’s a scenario. You have a fully stocked tank with no plants. You’ve settled in to changing your water (20%) once a month, sometimes every six weeks or so. Your water looks clear and your fish seem to be behaving the same as always. All is good. Then one day you notice a fish has died the previous night. It was old anyway. So you head to the store, buy a new fish. Your new fish kind of mopes around for a few days and then dies. “Must have been a bad fish,” you think. So you go to a different store, buy a fish, and it happens again. Thinking you should just play it safe, you do a 100% water change, just in case. Suddenly, all your fish are looking ill and some have died.

What’s going on here?

Well, one possibility is that because you wait too long between water changes, the level of nitrates in your water has steadily built to a high concentration. Your fish are accustomed to the high levels, because of its slow increase, and are getting by well enough. All the new fish you add to the tank are getting shocked by the high level of nitrates in your water, causing intense stress and ultimately death. “But what about after I changed the water? That should have fixed it.”

Well, because you didn’t change your water often enough, the pH started to trend lower and lower, because of biological processes. It became rather acidic over time. This is called “old tank syndrome.” Because it happened slowly, your fish got used to it, like with the high level of nitrates. When you suddenly gave them all new water, much higher in pH, they were shocked and stressed, just like your new fish. The stress left them open to illness and premature death.

I should also make clear that your old fish were not simply “getting used” to the high concentration of nitrates in the water. It’s not like they were toughening up to it and were doing fine. The long exposure to high nitrates were indeed doing them harm. It’s just that the gradual increase allowed for the damage to happen gradually. This left your fish weakened and further susceptible to the danger of suddenly and drastically changing the pH of the water. A crash was inevitable in this tank.

“I don’t need to change water because I have live plants.”

It’s true that live plants can be a huge asset to the aquarium. They help to remove nitrates and other unsavory compounds from the water. They’re great. The problem comes in when aquarists rely on plants to do all of the work.

What often happens is that we end up only topping off the evaporated water in the tank. Over time this can cause some problems, because water hardness can steadily rise, inhibiting some plant growth. When the plants are inhibited, they stop growing properly, therefore they don’t clean the water as effectively. They also can no longer fight off algal blooms as well as they used to. This in turn inhibits the plants further. Another problem that can occur is from the accumulation of suspended organic particles in the water. This can give the water a yellow tint, similar to that produced by a low concentration of tannins. These organic compounds filter the light passing through the water. Some plants that require higher lighting can be negatively impacted by this, further inhibiting their growth.

Do you plants a favor, change the water regularly.

Stocking and Feeding

“A small tank is good for beginners.”

Actually, the opposite is true. Smaller tanks are more difficult to maintain, because they small volume of water means that things can change rapidly. If ammonia is building up in a tank, it will take longer to happen in a larger tank than in a smaller one. This is also important for heating. If the power goes out in the middle of winter, a large tank will hold its ideal temperature for longer than a small tank will.

“Fish and turtles only grow to the size of their tank.”

This is a big one that I hear a lot. Animals grow to the same size whether they are in a pond or a ten gallon tank. I don’t know where this myth came from, but I suspect it is because fish kept in small containers usually die much younger than those in properly sized aquariums.

“Bettas/Goldfish should be kept in bowls.”

Firstly, goldfish should not be kept in bowls. End of story. They grow too big. Put your goldfish in a properly sized tank, according to the adult size of that variety, and provide it with adequate filtration.

While a betta fish has adaptations that allow it to survive in small vessels of water, this is far from ideal. Like any other fish, they prosper in larger volumes of water with proper filtration, which brings me to the next point…

“Bettas don’t like filters.”

This is one that I heard recently. I think it’s because sometimes filters can have flow rates that are too strong for the betta’s long fins. This can tire the fish out, causing stress. Certainly bettas benefit from having clean, filtered water, like any fish. Simply make sure the flow of water produced by your filter isn’t too much for your betta, and he or she will do wonderfully.

“Feed your fish as much as they can eat in two minutes.”

This is a very clunky and wasteful measurement for feeding fish. It’s spread by companies that make the food, because they want you to buy food more often! It’s very hard to gauge how much food that is when you are taking a pinch of flakes out of the container.

I recommend starting with a tiny amount. See how much the fish eats. Then try a tiny bit more the next time you feed. Soon you’ll see how much your fish can eat in one sitting (floating?). You also don’t need to stuff your fish with as much food as you can at one time. Try feeding a tiny amount a couple times a day. I always tell people it is better to under feed a bit than to over feed. This is because uneaten food can decompose, causing poor water conditions.

“A fish’s stomach is the same size as its eye.”

Okay, I’ve been guilty of this one, but I’ve learned the error of my ways! The size of a fish’s eye vs. stomach varies widely among species. There is no relation between the two.

“Snails are bad!”

An infestation of snails can be an eye-sore, but for the most part they are harmless. Snails generally do not eat healthy plant material. They do perceive when a plant is sick or dying. In those cases, they will happily eat the weakened or dead plant material. We see this and think that the snails are killing our precious plants! Snails actually provide a few great services in the aquarium. They eat leftover food, algae, dead plant matter, and churn the substrate.

Like I mentioned above, they do add to the overall bioload of the tank, and their population should be kept under control. And, like with live plants, don’t rely on them to do all of the work for you.

Fish Health and Care

“You need an air stone for oxygen.”

Air stones can be used to provide oxygen to the tank water, however they are not required. If you have a filter circulating your water that agitates the surface, then you have plenty of gas exchange going on. But hey, if you like of the bubbler, have at it Hoss.

“Distilled water is better than tap water for fish.”

Distilled water is actually not great for fish. They biggest problem is that distilled water contains no nutrients at all. It’s just H2O. Fish and plants need other nutrients to live and grow properly. It’s fine to use tap water, just make sure you are treating it with a dechlorinator and changing your tank water regularly.

“It’s okay to flush a sick or dying fish.”

You should never do this. It’s a cruel way to end the life of any animal. There are a few different methods for euthanizing fish, and each is better for different sizes of fish. If you want to humanely put down a sick fish, consult your local veterinarian or fish store. Please don’t simply flush them.

“It’s fine to release your fish into a local pond or stream.”

There are a number of reasons why this is a bad idea. First, you are likely sentencing your fish to death. Most aquarium fish are tropical, and unless you live in a tropical climate, your fish will not last long. Another reason why you shouldn’t do this is because you are potentially destroying a natural habitat. Your pet fish may be carrying parasites or some other illness that is going to cause problems for the local wildlife. Lastly, your fish might actually survive and reproduce! This is the worst-case scenario. There are many invasive fish species out there destroying natural habitats for native species.

Don’t be a jerk. If you can’t care for your fish anymore, call your local fish store. They may be glad to take it off your hands, or help you find someone who will.

“My house is warm / It’s summer, so I don’t need a heater.”

Your house may be quite cosy. But keep in mind that you wear clothes and sleep under blankets. You’re also warm blooded, which means your body can regulate its temperature automatically. Fish are cold blooded, which means their body temperature is the same as the water they are swimming in. Most aquarium fish are tropical species, which prefer water in the range of mid 70s to mid 80s.

You should also keep in mind that the air temperature in your home fluctuates throughout the day and night. If you keep an automatic heater in your tank, it will maintain a steady temperature for your fish 24/7. Fish generally cannot tolerate temperature swings greater than 2F during short periods of time. Swings greater than that cause them stress, and constant stress can lead to illness and death.


Title image credit: Photo by ChristinaEatsBrains

What other myths do you know of? Let me know in the comments below.

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