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Coldwater Fish Questions and Answers

Author: Richard Guest of Pets Parade Team

Can I mix all of the different coldwater fish varieties in the same tank?

Any of the "fancy" goldfish varieties should live happily together in the same aquarium, i.e. Orandas, Moors and lion heads will all mix together fine. It is best not to keep any of these fancy varieties in the same tank with more robust faster swimming types such as comets and common goldfish or species such as Bitterling or Rainbow dace. These faster swimming fish are generally too boisterous for the sedate fancy varieties and may out compete them for food and stress them with their constant activity. Biorb goldfish bowls great for beginners and adds style to your home!

How many coldwater fish can I keep in my aquarium?

The exact number of coldwater fish you can keep will vary with the type and size of filter that you use, the frequency and amount of water changes and the size of the fish. However, as a rough rule of thumb 1" (2.5cm) length of fish per 12 sq in (30 sq cm) of surface area is a generally accepted average stocking rate (don't include the tail, just the body length of the fish). So a 24" long x 12" (61cm x 30cm) wide aquarium would support a population of 24 1" (2.5cm) long fish.

How often should I change the water in a goldfish bowl?

In an unfiltered bowl you need to change half the water every week. Mix the replacement water in a clean container with some dechlorinator water conditioner and allow to reach room temperature overnight. Alternatively, add a small amount of warm water until the replacement is approximately the same temperature as in the bowl. Once a month remove 50% of the water and the fish to another container and thoroughly clean the bowl and gravel. Replace the fish together with the 50% of the water they were transferred in and top up the bowl as above. By only changing part of the water and ensuring the replacement is a similar temperature you will minimise the disturbance to your fish. Assuming you have a filter system fitted then changing 25% of the water every 3 weeks should be sufficient. Make sure you add the correct amount of dechlorinator / conditioner to the replacement water before topping the tank up. If you have an aquarium without a filter system follow the instructions as detailed for a goldfish bowl else where in this section.

How often should I feed my fish?

In an aquarium with a mature biological filter you can feed your fish 2 or 3 times a day, but only a small amount each time (the food should all be consumed in about 10 minutes). In an unfiltered tank or bowl feed only one small amount every two days, any more than this will probably lead to pollution of the water and illness or death in your fish.

Will my fish be all right without food when I go on holiday?

You can add a holiday block to your fish tank before you go away, this will slowly dissolve over a 10 - 14 day period and ensure the fish have a steady supply of food. Well-fed and healthy fish will be ok without food for a 2-week period anyway, the choice is up to you.

Do I need to have an air pump making bubbles in my tank?

No, a filter return that creates a water flow at the surface will add oxygen to the aquarium by agitating the water surface.

Do I need a filter in my aquarium?

It is perfectly possible to keep fish in an aquarium without a filter provided you maintain a very low stocking rate. Waste excreted by the fish will be diluted in the water and broken down by naturally occurring bacteria and by plant growth. Keeping too many fish will lead to a deterioration in water quality and will probably result in illness and death in your fish. A filter acts to break down waste products from the fish and maintains healthy water quality. Therefore if you install a filter system in your fish tank it will enable you to keep larger numbers of fish and improve their environment.

How does a biological filter work?

The fish in an aquarium excrete large amounts of ammonia as a by-product of their metabolism, they are in effect swimming around in their own toilet. The main function of the filter is to act as a sewage treatment plant for your fish, it does this by providing a large surface area for bacteria to colonise. These bacteria break down the Ammonia and convert it first into Nitrite and then into Nitrate, which is relatively harmless to goldfish. The filter is in effect a living organism in it's own right, hence the term biological filter. It takes five to six weeks for the necessary bacteria to become established in a new filter, during this period it is important to stock the aquarium gradually, adding a few fish every week. The filter must be operated 24 hours a day to supply the bacteria with a constant supply of oxygen and food. When the filter media needs cleaning it is important to only wash it with water taken from the fish tank, cleaning the filter with tap water containing chlorine will kill much of the bacterial colony.

How often should I clean my aquarium filter?

Internal power filters generally need cleaning once a month, external power filters may only need cleaning 2 or three times a year (when the water flow starts to slow). Remember that any sponge or other reusable filter material should be washed only in water that you have taken from the fish tank. Under gravel filters should be cleaned once every three weeks by using a gravel cleaner/ syphon when carrying out the regular water change. In heavily stocked aquariums or when large fish are being kept the frequency may need to be increased.

What is the best type of filter for a coldwater fish tank?

There are many types of filter and most types work effectively, in general an internal filter such as a Fluval Filter or an undergravel filter will be most peoples choice for a general community tank set-up.

External filters are popular for aquariums containing large fish as they can better handle the large quantities of fish waste created.

Do I need to keep any water treatments for my aquarium?

You should always have a bottle of dechlorinator / water conditioner to remove chlorine when carrying out water changes. It is a good idea to keep a bottle of parasite treatment such as "protozin" for treating out breaks of Whitespot disease or other common parasite problems that may arise and a general antibacterial treatment for any cases of fin rot or other bacterial damage. When treating small fish speed is of the essence, so once the problem has been correctly identified it is better to have the right treatment to hand than to have to make a special trip or wait for delivery.

What is the best way to introduce new coldwater fish into my aquarium?

Turn off the aquarium light, float the fish on top of the water in the unopened bag they came in for 20 - 30 minutes to allow the temperature of the water in the bag to slowly adjust to the tank temperature. The fish should then be ok to let out. Leave the lights off for another hour or two and feed the other fish to allow the new fish to settle in while your existing stocks are distracted. This advice applies to most hardy tropical fish species, where your water chemistry is similar to that in the dealers' tanks. For more delicate species or when there is big difference in water chemistry it is better to slowly mix some of your water in with the fishes transport water over a period of an hour or more. Advice on the specific type of fish purchased should be sought from the dealer.

Do I need gravel on the bottom of my tank?

The short answer is no you don't need to cover the bottom of the fish tank with gravel (unless you are using an undergravel filter) but most people think some sort of covering looks better and the fish seem to prefer it. Alternatives to gravel can be used e.g.: sand, but make sure they are non-toxic and won't affect the water chemistry. When growing on baby fish many people prefer to keep the aquarium bottom free of gravel as this makes it easier to remove waste when feeding several times a day and carrying out frequent water changes that are required when raising a large brood of hungry fish.

Why do I need a test kit for my aquarium, I can see the water is clean?

Aquatic test kits show up levels of fish waste in the water: Ammonia, Nitrite and Nitrate, all of which are detrimental to the fish, and in the case of Ammonia and Nitrite can be fatal at high levels. Because these substances are all dissolved in the water they are not visible and it is quite possible to have water that is clear and appears clean but which is extremely unhealthy for your fish.

Should I keep tonic salt in my aquarium?

Goldfish are fresh water fish and while a low level of salt may not appear to cause any problems, it is better to keep your fish in fresh water unless using salt for a specific problem such as the treatment of ulcers.

My fish are dying and there seems to be no sign of a disease, what is the problem?

There can be many different reasons for fish deaths that appear to have no cause but probably the most common is poor water quality. Test the water for Ammonia and Nitrite, these are both poisonous substances that enter the water from fish waste. Neither of these waste products can be seen, so using an aquarium test kit is the only way to make sure the water is safe for the fish. If levels are too high, reduce immediately with a series of 25% - 30% water changes and reduce the amount of food given, you may need to reduce stocking levels or install a larger filter to prevent the problem recurring.

How long should I leave the aquarium light switched on for?

8 - 10 hours a day is about right for the average coldwater aquarium.

Why don't my aquarium plants grow?

First of all make sure you have genuine aquatic plants, many species sold for aquarium use are actually "marginal plants" and will never grow satisfactorily under water, it is best to purchase a good book with a chapter on plant selection to ensure the best type are purchased. Assuming that you have proper aquatic plants the next requirement is good lighting, a bright fluorescent tube such as a "triton" light should help and the use of a light enhancer / reflector will improve matters even more. Regular additions of a good aquarium plant fertiliser will help and large specimen plants will benefit from a root tablet placed by their roots every few weeks. Many people find that their goldfish will quickly eat any plants placed in the aquarium, in this case the only way to keep a well planted tank is to use plastic plants.

My goldfish keeps swimming off balance or turning upside down, what's wrong with him?

Your goldfish may just be suffering from constipation, try feeding some live or frozen bloodworm, daphnia or brine shrimp once or twice a week as a replacement for one of the normal dry food feeds. Some "fancy" goldfish gulp in air as they feed at the surface, which can cause buoyancy problems, try feeding sinking pellets instead of floating food. All varieties of "fancy" goldfish such as Pearlscales and Orandas are prone to swim-bladder malfunctions, which will cause this type of problem. The swim bladder is an inflatable "sack" inside the fish, which they use to control their buoyancy by inflating and deflating. If an infection with bacteria is the cause of the problem then dosing the aquarium with Interpet Swim-bladder treatment may cure the problem. Often however the swim bladder is physically pressed on by the fishes' internal organs, which are more constricted in the body cavity of a "fancy" goldfish, that has been bred into an unnatural shape. In this case the addition of cooking salt at 1/2 ounce per gallon may alleviate the symptoms but there is no permanent cure. Your fish will probably live for a long time even if you are unable to cure this problem, as this is not usually a fatal condition.

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