How to keep fish successfully

This week we take a look at keeping fish – both indoors and outdoors.

Indoor fish

  • Lots of variety possible – cold water or heated tank? Fresh water or salt water?
  • Best to start simple, and then do more if you feel that you want to  later. So cold water, fresh water fish are best to start with.

The goldfish is the most popular fish but they often fall ill and die, so many people are put off: they shouldn’t be. The commonest cause of a problem is poor environment, especially poor water quality. Over 90% of disease problems are due to poor water quality; poor water quality causes fish to fall ill because there is not enough oxygen in the water due to overstocking and/or too many organic wastes.

Less than 10% of problems are due to introduced diseases/parasites or other causes: infections and/or parasites can be introduced into aquaria with new fish or plants that have not been quarantined properly.

Frequently, goldfish tanks go wrong within the first few months of setup. The water may look crystal clear and there may be no obvious reason for problems, but the goldfish just fall ill, with fungus, fin rot and death. Replacing the fish doesn’t work; the same problem recurs. The problem is caused by poor husbandry in the first place so it’s essential to get this right. It’s best to find a good aquarium shop and ask them for detailed advice

The four main issues affecting fish tanks are:

Stocking level

You need to measure the surface area of water and only put in the number of fish that this allows. In a coldwater aquarium, you should allow 60 square cm for each 1 cm of fish body length excluding the tail.  For example, in a 60 x 30 cm tank you can keep a total of 30 cm of combined fish body length, such as three 10 cm-long fishes or two 15 cm-long fishes.


There are different types available: for long term fish keeping success, it is essential to get filtration right, so ask for expert advice when setting up your tank.


Feed a good quality flake food. Overfeeding is the most common mistake in fish keeping – the basic rule is that any food not eaten up in two or three minutes is too much.

If you have a water quality problem, stop feeding until the problem is over, to reduce the level of waste in the water. Then start feeding again infrequently: one small feed every day to minimise uneaten food.

Goldfish can go for up to two weeks without food – for example, when you go on holiday, it can be safer to leave fish unfed rather than risk a neighbour overfeeding them and polluting the tank.

Water changes

As a rule of thumb change 25% of the tank per week every week. It’s best to use a test kit to monitor water quality. If you have a water quality problem, change 50% of the tank water every day until the level of nitrate falls to the correct level.

Use a water conditioner  to treat tap water, to remove the chlorine and any dissolved heavy metals.

Outdoor fish

This is a topical subject for late spring: mid-March to July is the best time of year to introduce new fish, giving them plenty of time to settle in and put on weight before the onset of winter.

During the winter the fish enter a dormant state and stay near the bottom of the pond where it is warmest and feed only occasionally. It’s often best to buy the fish from a local supplier, so that they don’t have too far to travel to get to their new home with you

Look for alert fish that are lively with intact, erect fins and clear eyes. Any with spots or growths should be rejected, as should any apparently healthy fish in a tank with dead or sickly looking ones.

When you have picked your fish get them home as quickly as possible and float them in their plastic bags for about half an hour, out of direct sunlight. Then open the bag and mix in a little of the pond water, hold them for a few minutes as the water mixes, then release them.

What type of fish?

For small to medium size ponds the Goldfish (the same as the indoor variety) is a good choice. These are inexpensive and hardy, and can live in a pond with no filter or moving water.

For medium to large ponds Koi Carp and Golden Orfe are possibilities. These have more elaborate requirements, needing a pond filter and some moving water. They can be expensive fish to buy, and their care can be quite specialised, requiring significant investment to set them up properly.

Feed a good quality pellet, flake or stick food a couple of times a day in the summer months, giving much less food in the winter.

In the spring and early summer, watch out for herons, which can easily empty a garden pond of all its fish. Use a net on top of a pond heron scarers including flashing lights, bird of prey sounds, and even electric fences.

Koi keeping


Koi are at the top end of outdoor fish keeping. You can spend as little as €20 for a Japanese Koi, or as much as €5000 for a premium, show-winning 24 inch long three year old. They can live for sixty years if they’re looked after properly. In Japan, one Koi fish was sold for $200,000 – it’s a passion, not a hobby.

Koi keepers have shows, like dogs shows: the all-Ireland Koi Show is held in Antrim every August. If you’re considering Koi, you should first of all talk to someone who keeps them, visiting their premises so that you can see for yourself what’s involved.

To find out more, visit You’ll find useful discussions and advice from Koi keepers, including:

  • how to avoid problems during the design stage of building a pond,
  • building techniques (not all ponds are just a hole in the ground),
  • how to filter the pond water to provide good water for koi,
  • the type of pumps which are most suitable for circulating the pond water through filters or down waterfalls,
  • what to look for and avoid when buying koi,
  • how to treat koi which are sick, and
  • koi nutrition

Fish stockists in Ireland

  • between Tallaght and Blessington have a wide range of fish. Very helpful place to get advice as well as to buy what you need
  • in Portrush, Co Antrim, have wider range of Koi including bigger ones

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Please note that I am unable to answer veterinary questions in comments. If you have questions or concerns about your pet's health it is always better to contact your vet.

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