Things fishkeepers should not do

New to fishkeeping? Here are some things new fishkeepers shouldn't do...

Copyright © Practical Fishkeeping

The most important consideration when keeping fish is the fish themselves. These live animals are totally dependant on you to keep them alive and healthy. They demand the right care and respect, and it is your duty to offer them the best conditions that you can.

Don’t stock too quickly

A common no-no. If you stock your tank too quickly, there will be insufficient bacteria to cope with the amount of waste being produced, and ammonia and nitrite will build up to toxic levels.

When you have a newly set-up tank, the choice in aquatic shops can be overwhelming. It is all too tempting to fill it with fish within days, but you must resist and cycle the tank first.

Cycling means maturing the tank by leaving it for days or weeks before adding fish.

In this time, beneficial bacteria will increase in number, covering all the surfaces inside the tank from the glass to the gravel to the filter media.



Bacteria can be added when you first set up the tank, giving it a kick-start into maturation.

Maturation products will either add live bacteria direct to the water or create a flock which offers a slimy home where the bacteria can live and multiply.

While bacteria are growing, water quality can change rapidly from being clear of ammonia to being high in ammonia and then nitrite.

You must not add fish within this time because they will probably die, poisoned by the ammonia and nitrite – yet the bacteria rely on ammonia produced by fish to increase in number.

Ammonia is food for bacteria, but toxic to fish.
Fishless cycling is another way of feeding the bacteria with ammonia, but not exposing fish to it.



Raw ammonia (available from DIY stores and some chemists) can be added daily to feed the bacteria and make them grow in number before you add any fish.

The point at which they consume all the ammonia every day, leaving none behind, is when you add fish.

Another way of doing it is to add Waterlife Biomature or its equivalents, available from aquatic shops. A new tank takes at least six weeks to fully mature, and should be stocked slowly in that time.

Don’t overfeed

Overfeeding is one of the biggest causes of fish deaths. The type of fish that you keep will determine how often they should be fed.

As a general rule, small fish like tetras and Guppies need feeding small amounts of food throughout the day. This can be as much as three times per day if your filtration and water changing regime is good.

Larger fish can be fed less often, and once or twice per day is fine. Very large fish (60 cm/24”and over,) should be fed every other day.

There are other considerations, and they include how active the fish is, and whether they are herbivores or carnivores.



Herbivores gain little nutrition from the plant or algae matter that they eat. As a result, they have to munch on it constantly. Mollies and mbuna cichlids from Lake Malawi are herbivores.

Carnivores are adapted to deal with large, protein-rich chunks of meat, but eat it far less often. Predatory fish that eat other fish should be fed once or twice a week on large, fishy foods. Oscars and Piranha are carnivores.

Don't leave the lights on too long

A common cause of algae. Phosphate, ammonium and nitrate aside, if you experience a lot of algae, you may simply have the light on for too long.

Lighting is developed to be brighter than ever before, and is aimed at encouraging a particular response from your plants or corals, or to enhance your fish colours.

Plants and corals typically need the light on for around 10 hours per day, but only really need full spectrum (all your lights) for six hours per day. Anything more than this and it may cause algae.

If you are using bright lighting for plants, make sure that you have sufficient plants to out-compete any algae.



This can mean 70% coverage in your tank. One or two bunches of plants, plus 10 hours of light daily, mean algae, so either increase their number or use plastic plants instead.

Fish aren’t really bothered about lighting, and most would prefer it subdued. If you don’t have live plants or corals, cut the lighting right down so that you just have it on when you are at home.

Four or less hours per day will make it difficult for any algae to grow, and the fish will be fine in ambient room light at all other times. Never leave the light on 24/7.

Don't forget to acclimatise your fish

This causes lots of confusion and you will hear differing opinions depending on where you shop.

When you buy fish from a shop, you must tell them how long it will take you to get home so that the fish can be packed accordingly. Always take them straight home.

Once home, turn the aquarium lights off. If the fish have been packed in the dark for several hours, expose them to the room light slowly so as not to shock them.

Float the unopened bag in the water for about 20 minutes to allow the water inside the bag to attain the same temperature as the water in your tank.

If the bag is sealed with a rubber band, remove it and roll down the sides until it floats freely on the surface of the water. If the bag is tied, cut the knot off with scissors and then roll the sides down.



Next add some water from the aquarium to the bag. This will slowly alter the water chemistry. Continue over the next ten minutes or so until the bag water is mostly tank water.

Gently catch the fish with a small net and release them into the tank, then discard the bag water, as it may contain ammonia.

Observe the fish over the next few hours and days, making sure that they settle in ok. The ideal is always to quarantine new fish for a couple of weeks. This makes sure that they are healthy and feeding before adding to the main tank.

Don’t mix the wrong fish

New fishkeepers tend to want a large variety of colourful, different looking fish, but not everything in the shops will be suitable.

Choose a good aquatic shop with informative labels and always ask for advice. Write down the make and model of your tank, and how many litres or gallons it holds.

Do your own research as well as asking advice from retailers. Read books and PFK to give you an idea of the sort of fish you wish to keep, and try the hardiest fish first.

Don’t be tempted by juveniles of big fish. If your tank won’t house them at an adult size, don’t buy them.



Aim to keep community fish first, as these often stay small, and are easy to keep. Non-community fish may be large, predatory, aggressive, or all three, and are not good for beginners.

A community of small fish offers you the most choice, and you will be able to fit more of them in your tank than any other fish.

Don't go for the cheapest

There are some things that are vital to the success of the tank. The filter, heater and test kit must be reliable or it could mean disaster.

Filters should be man enough for the job in hand, and capable of doing a bit more, too. Choose a filter with a large media capacity and multi media options.

For peace of mind, install two filters so that if one stops, the other will keep your fish alive. Two filters are also safer when it comes to maintenance as they can be washed alternately, keeping bacteria levels high.



The need for an accurate heater is obvious – most long-term fishkeepers have come across either boiled or frozen fish.

To err on the side of caution, fit two smaller heaters so that if one sticks on, it will not heat the water too quickly and you can do something about it.

When choosing any heater, make sure that it has a wide-enough temperature range to accommodate all the fish that you wish to keep. Discus need a heater that can heat up to 32°C/90°F.

Those that are controlled by microchip are more accurate than the antiquated, bi-metallic strip method of temperature control.

Several PFK product tests have revealed that not all test kits are equal. Many illnesses can be traced back to continual, low level ammonia and nitrite in aquarium water. A poor test kit won’t detect it.

Don’t overstock

With the exception of Malawi cichlids, you should never put too many fish in your tank. It is all about bacteria, and if there isn’t enough to break down all the fish waste, the fish will poison themselves with their own ammonia.

Overstocking most tropical fish can cause aggression, disease and stunt growth.

Oxygen will be depleted through respiration and your tank will be resting on a knife-edge.



One more fish and the biological filter may fail to cope, or your mechanical filter may clog, stopping the filter and killing fish.

Stocking levels do vary, and it used to be calculated by working out the surface area of the tank.

Nowadays with better filtration it is hard to give an exact figure, but PFK recommend the following:

Tropicals: 1” per gal/ 2.5 cm per 4.55 l initially, then up to a maximum of 2” per gal/5 cm per 4.55 l after six months.
Coldwater: 1” per gal/2.5 cm per 4.55 l.
Marines (fish and inverts):
1” per 4 gal/2.5 cm per 18 l.
Marines (fish only): 1” per
2 gal/2.5 cm per 9 l.
Ponds: 10” per 100 gal/25 cm per 455 l.

Don’t stop reading!

From the moment you first take up fishkeeping, your education will begin, but don’t worry – it’s not like school and is much more fun!

You will undoubtedly start off as a general fishkeeper, trying a bit of everything including all sorts of fish and plants. Instead of using trial and error, the best way to gain experience is to read as much as you can in books, the Internet and PFK.

Every expert was a new fishkeeper once, and a lot of their knowledge comes from studying their favourite fish in books and magazines.

As your experience grows, you may wish to breed your fish, and again by reading up on how to breed a particular species, you can make it happen.


Many of the world’s experts are simple hobbyists who read everything they could, applied it and then added their own experiences to what they read.

You can become an expert in a very short time in fishkeeping, and being the best at what you do can be very rewarding.

If you need further advice, please sign up to our forum where other fishkeepers will be happy to help you with your problems.