Maintaining Your Freshwater Aquarium (2021 FAQ Part 4)


How many fish can I have?

There are a number of formulae which give a guide as to the number of fish a certain size aquarium can hold, but there is no one hard and fast rule. The number of fish will depend on a number of factors, the most important being:

  • Size and type of fish – larger fish clearly need more room than smaller fish, but the type of fish is also important. Some cichlids, for example, produce a lot of waste product, more so than other fish of similar size, and are also territorial so need more space per fish. The bodyweight of a fish is more important than its length in determining the room required, and body weight increase exponentially with length. Therefore, a large stocky fish needs more water per cm/inch of body length than a long and thin fish.
  • Filtration – the better your filtration, the more fish your aquarium can support, at least to a certain point. Biological filtration is the most important consideration here. (see our filtration information pages for more details)
  • Size and surface area of the aquarium – water volume is important, but remember that gas exchange happens at the surface of the aquarium, and so the surface area may be a limiting factor in determining the capacity of some aquariums. Surface area is less important where there is good filtration and aeration. A rough guide to the number of fish is to allow approximately 1 litre of water per centimetre of fish where there is good filtration. Allow 2 litres or more per cm for small to mid-sized goldfish and cichlids, or where filtration and/or surface area may be limiting. Allow more water (3-4 l per cm) as the fish grow larger, or where they are territorial.

Judging the number of fish a tank can hold is an ability that comes with experience. There are some tricks that may help, however. It is always a good idea to add fish gradually when stocking a new tank. With each new fish you add, watch carefully for any signs of stress, both in the new and old fish. If the fish do seem stressed, or if disease strikes, you have probably reached (or possibly exceeded) the capacity of your tank.

Testing your water can also help to determine when the capacity of your tank has been reached. Check the pH of your water regularly. If this stays at or near the pH of your source water (or around neutral) then you can safely add more fish. If it drops very rapidly this is a sure sign that there are too many fish or that the fish are too large for the tank. You can also monitor the level of nitrate in your tank. If this stays at or near zero (with zero ammonia and nitrite), you can safely add more fish. If it rises steeply, then your tank is probably overcrowded.

If you find that whenever you add a new fish that an older fish dies, this is again a sign that the maximum capacity has been reached, but it is best to determine this by less destructive methods if possible!

What fish are compatible with one another?

In coldwater, there are a limited number of species available, and most are compatible. Goldfish may grow large enough to eat some small fish (eg danios, white clouds) but they are not aggressive – only a fish that they can easily swallow would be eaten. Hence, you can mix larger goldfish with smaller goldfish, and with any other fish larger than the gape of their mouths. Some coldwater fish can be aggressive. Giant danios and rosey barbs are very active swimmers, and may nip fins. They should only be kept in larger tanks, and with other fast swimming tank mates (eg comet goldfish, danios, but not fancy goldfish or fantails). Paradise fish are territorial and may be aggressive, particularly toward one another, but in a larger tank they can be mixed with fast swimming fish. Australian natives for coldwater are mostly large and aggressive and do not mix with other species, but there are a few types that do. For more information, see our fish species guide. Since there are literally thousands of tropical species available, it is impossible to provide compatibility information on every possible combination of fish. There are, however, a few general rules. Fish that are peaceful and do not grow too large and are not overly territorial, predatory or aggressive, are commonly termed community fish. Most community fish are compatible, however, very small fish (such as neons) should not be combined with larger community fish (such as angelfish and gouramis) which may consider them as food. Stick either with a combination of small to medium fish, or with one of medium and larger fish. Fish which generally do not mix well, do so for a number of reasons. Firstly, they may grow very large, and, even if peaceful, most larger fish will eat small fish (ie those that fit comfortably in their mouths) if given the chance. Mix these fish only with other large species, in a tank with sufficient room. Secondly, they may be predatory, and specialize in eating other fish. A predatory fish may not necessarily be aggressive, ie they may eat only smaller fish that they can swallow whole, but many predators have very large mouths for just this purpose! Again, mix these only with fish large enough not to be eaten. Some fish may be predatory and aggressive, ie they will attack and kill fish as large as themselves – or even larger – for food. These fish can only be housed with fish that are either too fast to be caught, or which are able to defend themselves. Other fish are territorial, that is, they will pick a spot in the aquarium and defend it against other fish. Some (eg red tail sharks, rainbow sharks) do this by chasing away other fish, but usually without causing any real damage. These can be mixed in a community tank, but not with very slow moving, placid or easily stressed fish. Some fish (eg many cichlids) defend their territories much more agressively, and these should only be mixed with other fishes of similar temperament, in a tank large enough for each to establish its own space. Some fish are not compatible for the opposite reason, they may be shy, difficult feeders, that can be stressed simply by the normal boisterous activity of community fish. Discus are the prime example here. Fish like these should only be mixed with small, placid species. Finally, some fish are not compatible because the water conditions they need are not suitable for other species. Primarily these are brackish species, which need alkaline water, ideally with salt added. For more information on the compatibility of fish, look up the species you are interested in in our species guide.

Will my fish breed in the aquarium?

In most cases, the answer is no, but there are exceptions. Most fish are egglayers, and will not spawn when crowded in a tank with a lot of other species. Even if water quality is good and fish do spawn, the tiny eggs are usually quickly eaten in a community tank. Livebearing species (platies, mollies, swordtails, and guppies) are however prolific breeders and are not put off by the presence of other species. As their name suggests, rather than laying eggs, these species give birth to live young after the female is fertilized internally by the male. Although free swimming and capable of taking finely ground-up flake food, livebearers young are very small, and will often get eaten in a community tank. If you want to raise the young, you will need either to move them to a different tank, separate them with a breeding trap or net breeder in the main tank, or provide them with plenty of shelters, eg, lots of plants in which to hide. Many cichlids may also breed in the aquarium even in the presence of other species. They will only breed if you have a mating pair (cichlids will select partners and pair off, simply having a male and female in a tank does not guarantee that they will breed), and they are happy with the water conditions. Cichlids lay eggs, but instead of scattering them and leaving them as many fish do, cichlids will lay them in a safe place (eg on a rock or in a cave) and defend them. Some species even take the eggs into their mouth to brood them. If the cichlids are good parents, and there are not too many other fish in the tank, they may raise the young with very little help. However, the parents may become stressed by the presence of other fish, or if they are simply not good parents, they will eat or abandon the young or eggs. Often the young fish, even with the help of their parents, will not be able to obtain enough food and will starve. For these reasons, it is best to use a separate tank to raise the young, either with or without their parents. As mentioned, other types of fish may spawn in a community tank, but this is very rare. However, if your fish do lay eggs, it is a sure sign that they are happy! Eggs will generally be eaten very quickly, if you want to breed the fish, you will need a separate tank.

How can I sex my fish?

Some fish show extreme sexual differences – the males and females may even appear to be different species! On the other hand, some fish show absolutely no external sign of their sex. In many species, there are subtle signs that can be used to sex the fish, but there is no rule that applies across all species. Some general rules (to which there are many exceptions) are: Male livebearers have a gonopodium, a modified anal fin that is rod-shaped rather than fan-shaped. Females have a fan-shaped anal fin. (Livebearers include guppies, mollies, swordtails & platies). Males of many species, eg many barbs and danios, guppies and some tetras, are smaller and more colourful than females. Females will often be plumper. Males of some species have longer fins, particularly the dorsal and/or anal fin, eg gouramis, some cichlids. Some have extensions of the caudal (tail) fin, eg some tetras, other cichlids. In some cichlids, males and females are different colours or show different patterns. In others, only the males are colourful, while females are drab or grey.

What’s the best way to clean my aquarium?

Tank maintenance is best performed on a regular basis. If the aquarium is left too long before a big clean-out, a drastic change in water conditions can occur. This causes immense stress on the fish and can kill them. WITHOUT A FILTER If you do not have a filter, it is best to change between 50 and 80% of the water once a week. Leave the fish in the tank and scoop or siphon out the water to be changed. Replace the water with fresh tap water treated with a chlorine neutralizer. In cold weather, it is best to let the new water stand for a while so that it warms up to room temperature. Do not use water from the hot tap or that has been heated in a kettle. These contain dissolved gases and metals which can harm or kill the fish. Once a month (or more often if necessary) clean any algae off the sides of the tank, and once every two or three months take the fish out into a separate container (in some of their tank water) and give the gravel a rinse out with water from the aquarium to remove solid wastes. Alternatively, perform at least every second water change with a gravel cleaner siphon. WITH A FILTER Ideally, remove and replace approximately 20 – 25% of the water every two weeks. Many tanks can be left longer, but you should not leave any longer than a month between water changes if possible. Use a gravel cleaner siphon so that you need never strip down the aquarium. Leave the fish in the aquarium while cleaning, but turn off your filter and heater to ensure these do not run dry. In cold weather, let the new water reach room temperature before adding it to the aquarium. This may still be below the desired temperature for tropical fish, but since only 25% is being changed, this will not dramatically lower the temperature. Do not use water from the hot tap or that has been heated in a kettle. These contain dissolved gases and metals which can harm or kill the fish. As necessary, clean algae off the sides of the tank and clean the filter media. Mechanical filtration media should be rinsed out every 2 – 4 weeks. Chemical filtration media should usually be discarded and replaced after 1 – 2 months. Biological filter media should be disturbed only if they are becoming clogged with detritus and then they should only be rinsed in water that has been taken from the aquarium.

How do I stop the water from going cloudy or murky?

If everything is as it should be, your water should not become cloudy, murky or smelly. By far the most common cause of cloudy water is overfeeding. If your water is murky test for ammonia. The presence of ammonia indicates that organic matter (such as uneaten food) is decaying in the aquarium. Perform a water change, clean up any excess food (or dead fish/snails/decaying plant matter), and check the fish for any signs of stress. If no ammonia is present, then the cloudiness may be caused by a number of things. In a newly set up aquarium there may be some cloudiness caused by dust from the gravel. This will settle in time and this settling can be hastened by using a water clearing agent. Also in a relatively new aquarium, or where new filter media has been added, a bacterial bloom can occur. This bloom is of beneficial filtration bacteria. Their numbers will settle down in a few days. Very rarely, cloudiness can be caused by suspended algae. If cloudiness does not disappear in a few days or when a water clearer is used and no ammonia is present, perform a water change and then try an aquarium algicide.

Can I convert my coldwater aquarium to tropical?

The main factor in determining this is the size of the aquarium. In order to maintain a stable temperature, a tropical tank should be around 25 l (eg 16″) or larger. If you do not have a filter in your coldwater aquarium, you will need to add one before you convert to tropical. If you have a suitable size aquarium and filter, then all that is needed is a heater and thermometer. You may also wish to add a light, but this is optional. You can keep your coldwater fish if you convert to tropical, but there are a few things to bear in mind. Goldfish will grow more quickly when kept at warmer temperatures, and they can be quite messy. For these reasons they do not mix well with many tropical fish. Most other coldwater species are good tropical community fish and pose no problems.

Can I convert my freshwater aquarium to marine?

Converting an aquarium from freshwater (either coldwater or tropical) to marine is usually a large undertaking. Marine fish or reef keeping usually requires a larger tank than either coldwater or tropical, although it can be done in certain smaller aquaria such as all-in-one units, in most cases a minimum of 80 to150 l is recommended. The amount of filtration required for marine is greater than that necessary for most freshwater aquariums, and filtration should be primarily biological. This is not to say it cannot be done, but the investment in new equipment may be significant. If you want to set up a freshwater tank that can later be converted to marine with minimal expense, you should invest wisely in a suitably sized aquarium, excessive (for freshwater) filtration, and a good quality heater. Read through our marine FAQs to learn more about what is required for marine aquariums.

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