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Common Fish Keeping Myths

In this article, we are going to try to unravel some of the more common myths about keeping fish as pets. This guide is aimed at beginners with the goal of dispelling the myths and providing insight into how they came about. Believing these false tales of aquarium advice will harm your aquarium fish and present other challenges for you to overcome.

Fish Only Grow to the Size of the Tank

In one way of thinking this is true, but in a very bad way. A fish kept in a tank that is too small for it may become stunted and this may end up with serious internal problems with its skeleton and internal organs. The end result for many stunted fish is early death, but prior to that, they are stressed and more prone to disease.

Leaving a New Tank to Stand for a Period of Time

It is a common myth perpetuated by many fish shops that you should leave a new tank to stand for a few days, a week, two weeks, or more although it has no factual basis at all. Many local fish stores call it a settling period, a cycle, or even a fishless cycle. Unless there is a source of ammonia in the tank, then there will be no growth of a beneficial bacteria colony and therefore there is no benefit to leaving a tank to stand in this way. Also, note that a pinch of flake fish food or adding a frozen prawn is not sufficient to provide the quantity of ammonia required to establish a bacteria colony large enough to support a full tank stock. You should ignore this myth or brush it off if someone tries to guide you with this false information. 

Carbon Filters

There is no need to run carbon filter pads in your filter all of the time despite what the instructions in your new filter manual will tell you. Carbon is used to remove unwanted external chemicals from your tank and therefore the most common use of carbon is to remove medication from a tank after a course of treatment.

There is also the fact that carbon absorbs chemicals, but these absorbed chemicals can be replaced by other chemicals that are more readily absorbed by the carbon at a molecular level. Therefore, there is a risk that old carbon in your filter can actually start leaching unwanted chemicals back into your tank. It is worth keeping a sealed carbon filter sponge in your fish cupboard for emergency use

Changing Filter Media Every Month

Again many instruction manuals for new filters suggest you replace filter sponge media every month / 2 months / etc. If you consider that the beneficial cycle bacteria live within these filters, carrying out this suggestion will result in your filter cycling every month and therefore exposing your fish to toxic chemicals regularly.

There is no need to replace your filter sponge media until it is literally falling apart in your hands (this will usually be at least 10 years and is generally longer than the life span of your filter pump). To maintain your filter sponge media simply rinse half of the media in used tank water every month or so or whenever the flow through the filter is reduced.

Changing Filter Wool (Filter Floss)

As with filter sponge media, filter floss does not need to be replaced until it is physically falling apart when it is handled. The primary purpose of filter floss is to filter solid waste from the water. Therefore, this media is what will contain most of the gunk that we get on our tanks and will therefore require rinsing more often than the sponge media.

There are very few friendly bacteria residents within this media, so there is little harm in replacing it periodically, however it can be regarded as an unnecessary expense rather than the necessity that your filters manufacturer will often state in the manual. If you do choose to replace this media, then it is possible to buy sheets of filter floss and cut and fold it to size in order to fit inside your filter housing rather than paying for expensive branded media.

Nitrate Removal Sponges

The cheapest most effective way to remove nitrates from your tank is to change the water. The effectiveness of these sponges is open to debate, however, they should not be considered a suitable substitute for regular water changes, as nitrate is not the only unwanted chemical that is removed from a tank during a water change. 

There are many other chemicals and hormones that your fish excrete that we do not test for but are best removed from the tank via water change (Waalsted theory excepted, but this is not for beginners). If you do have these sponges in your filter, then there is no problem with just leaving them in as if they were additional bacteria housing sponges. So, they do not need to be replaced until they are physically falling apart in your hands as detailed above.

Water Changes and Tank Maintenance

As discussed above, maintenance of your filter simply requires that you rinse out the filter media in used tank water to remove the gunk. You do not want to be too vigorous with the cleaning as this may result in the loss of beneficial bacteria.

One significant myth is that large water changes are bad. This is only true if there is a large difference between your tap water statistics and your tank water, however, if you keep your tank water close to the status of your tap water then there is no harm in carrying out very large water changes even above 80%. You should note that for many fish species, the river that they originally came from often have water changes of 100% every second even in relatively slow moving rivers and similar bodies of water.

If there is a difference between your tank water parameters and your tap water, then it may be worth starting a new thread to get advice on how to proceed, however if there is no ammonia present, then 20 – 30% water changes on a daily basis should not be a problem.

Bottled Bacteria Supplements

There are many products on the shelf that claim to aid or establish the development of the beneficial bacteria colony. These products often claim to contain the beneficial bacteria; however they are usually stored at room temperature on a shelf in the fish store for months at a time and have use by dates in excess of a year. This begs the question, how is a living bacterium going to survive in a closed bottle with no food or oxygen source for such a long time period? The simple answer is that it can’t and in the main the products are worthless.

There is currently only one product reported by reliable users of this web forum that has any worthwhile effect, it has a very short shelf life and must be stored and transported in a refrigerated environment and is only available in North America, its shelf life and transport requirements are such that it cannot be exported to Europe or other parts of the world.


We strongly suggest that you read this nitrogen cycle article to start to get a good basic understanding of the process to help you understand what it going on in your tank.

While most people refer to it as cycling a tank, you are actually cycling a filter. It is not possible to cycle a filter without a source of ammonia, therefore when the local fish store suggests that you fill a new tank with water and leave it for a week to “cycle”. This is flawed thinking and it will not cycle as there is no ammonia in the tank. There is virtually no beneficial bacteria in the water column, therefore getting tank water from somewhere else does nothing to help.

Many local fish stores will suggest that you should not carry out a water change during a “fishy” cycle. If you can measure ammonia on nitrite using your test kit, then there are more of these chemicals in the water than the current bacteria colony can consume. The bacteria colony will reproduce until it no longer has an excess of its food supply. Carrying out even a very large water change will not completely eradicate the excess bacteria food from the water and the bacteria colony will grow whether a water change has been carried out or not. Obviously diluting the concentration of toxic chemicals will have a significant benefit to your fish and as we have explained, will not slow the growth of the bacteria colony by any significant measure.

You Need an Air Pump

Oxygen is transferred to the water at the surface. Anything that breaks up the surface of the water gives extra oxygen transfer and will increase the oxygenation of the water. An air pump does this, but it is unnecessary as simply pointing your filter outlet towards the surface of the water will increase surface agitation and oxygen transfer. You can install an air pump for aesthetic reasons if you want to, but don’t believe the myth that it is required.

You Must Have a certain pH Value

Fish are entirely unaffected by a pH within the range of 5.5 to 9.0. In actuality, the range could very well be wider than that. Filter bacteria efficiency reduces as the pH level drops. The filter bacteria will go dormant and die off before the fish start having any problems due to pH

Fishkeepers who have considerable planting and CO2 injections systems often report changes in pH by over 1.0 within a few seconds of the CO injection system operating with no ill effects to the fish, thus proving that fish are able to cope with very large changes to pH several times every day.

Of more importance to the fish is the Total Dissolved Solids (TDS) in your water. TDS is usually related to hardness of water and hardness is often indicated by pH, but they do not always have to correlate. Fish that are known as soft water low pH fish are in fact intolerant of high TDS, and conversely fish that are known as hard water high PH fish are actually intolerant of low TDS. It is possible to purchase a TDS meter, this is generally considered unnecessary, as the pH and water hardness relationships are usually true. That means that a safe estimation of TDS can be made from measuring pH, GH and KH.

The use of the pH up / down / all-around type products is not recommended and can cause more problems than they attempt to solve. While the pH is unimportant, adding unnecessary chemicals to your tank is never a good idea.

You Need to Add Aquarium Salt

Two points here:
1) Aquarium salt is actually just an expensive version of ordinary table salt. So, there is nothing wrong with buying the significantly cheaper version from your local supermarket

2) Many local fish stores advocate adding salt as a general tonic to prevent disease. However, salt dipping should only be carried out for very specific purposes and at a much higher concentration than you would add to a community tank anyway. Adding salt to a community tank will have a significant negative impact to many intolerant species which are very common in many hobby aquariums. You also run the risk that disease or parasites could develop a resistance to it.

One of the main beneficial uses of salt is to counter nitrite poisoning, where salt ions will out-compete the nitrite ions for absorption into the fish’s bloodstream. While this is not a substitute for water changes to reduce the concentration of nitrite until the cycle bacteria catches up with the excess nitrite food supply, it is worth considering as a temporary measure while there is excess nitrite present. The typical dosage for this is half a teaspoonful per 10 US gallons of water.

Snails are Bad

Snails usually come into your aquarium as eggs on plants and it is almost impossible to stop them. While they can look unsightly, they contribute very little to the biomass of the aquarium and can be virtually ignored from any estimation of stocking capacity. It is possible to argue that the snails offer a positive addition to the aquarium as they consume any uneaten food and also consume other waste within the tank.

Getting a fish, such as a loach, to eradicate snails is not a good idea, as you may end up with a fish that is not appropriate for your aquarium. Plus, as snails often live inside the substrate and the filter housing the fish will not be able to complete the job that the local fish store suggested it would anyway.

If you want to try to prevent them from spawning, there is a suggestion that soaking new plants in a copper container for a few days will eliminate them, as snails are not tolerant of copper. In theory, this is true, although we have no guidance on how long is required to kill off all snail eggs on any new plants.

Algae Eating Fish to Keep Your Tank Clean

Many local fish stores will suggest that algae eating fish are required as they will keep your tank clean. Further, they will advise you as scavengers they do not contribute to tank population and should be ignored from capacity calculations. This is untrue as no fish will completely eradicate your aquarium of algae, therefore they will never represent a substitute for elbow grease and a regular maintenance regime. You will also have to add supplementary food for these fish and they still produce fish waste. Finally, they also need oxygen to survive and, as such, cannot be ignored from any stocking capacity.

White Spot is Caused by Stress or White Spot is Present in Every Tank

White Spot or Ich is a parasite that has a lifecycle. First, it is not present in every tank. Second, while stressed fish are less able to fight off a parasite attack as well as healthy fish, they cannot get White Spot unless the parasite is present in your tank. The parasite cannot survive for long in your aquarium without a host fish to feed on. This is why it is not present in all aquariums, as newly setup aquariums without fish are certainly clear of the parasite and even a used aquarium that has been empty for a relatively short amount of time will be clear.

The White Spot parasite usually enters your tank when an infected new fish is added to your aquarium, which is why most local fish stores like to perpetuate the myth that it is present in all tanks, as it deflects the blame away from the fact that they sold you a fish that was infected. This is one of the many reasons why quarantine tanks for new arrivals is so important.

Beneficial Bacteria are Found in the Water Column

As we stated before, the beneficial bacteria that is part of the nitrogen cycle live on surfaces.  Therefore, as the media within your filter is designed to have a very large surface area, most of your bacteria live within the filter media. As the water in your tank does not actually have any hard surface for the bacteria to grip onto it has no bacteria present and is therefore worthless when it comes to cycling a new tank.

Guppies / Platys / Neons are Good First Fish to Cycle a Tank

We would strongly suggest that you carry out a fishless cycle rather than cycle a tank with fish. However, many local fish stores will suggest that you add “hardy” fish to cycle your tank and then many of them point you towards Guppies and Platys. 

Due to line breading for specific color strains these fish can certainly not be described as hardy anymore, with fancy guppies particularly fragile nowadays. Indeed, Neons and a few other common species do better in a more established tank, therefore you should do your research before you buy your fish to ensure full compatibility within your tank.


We hope that we have provided some insight as we dispelled many of the most common and popular myths about keeping fish.

Without a doubt the three most common mistakes are letting a new aquarium stand for any period of time, the fish will fit the tank myth and not understanding the nitrogen cycle.

Finally, it’s always a good idea before you start keeping fish that you have researched the topic thoroughly and have chosen the species of fish you want to keep. 

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