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Coldwater Fish in a Nutshell

Welcome to keeping coldwater fish!  We hope this quick introduction will put you on the right track so that you enjoy your fish as much as we do.

Thankfully the old-fashioned idea of keeping fish in stagnant bowls has been replaced with an exciting variety of filtered tanks to suit every home and budget.  These days, keeping coldwater fish is far easier and more rewarding (for us and the fish!).


What are coldwater fish?

Coldwater fish are fish that can live in unheated or ‘room temperature’ water at about 18oC.  The most common choice are goldfish, but for smaller tanks there are lots of other fish to choose from that won’t grow so big, like minnows and danios.  Some types of fish, like   platies, live in sub-tropical or temperate conditions of about 22oC so would be ideal if the tank is in a warmer room.

What size tank do I need?

There is an old wives tale that “fish will only grow to the size of the tank” which sadly came about because fish kept in small tanks rarely live long enough to become adult size!  You should choose a suitable size tank for the space in your home and the adult size of the fish you plan to keep.  An adult goldfish should grow to at least 15cm.  A 30 litre tank is the minimum recommended for  10 minnows/danios or a young goldfish (but you will need to upgrade to a larger tank after a couple of years).  A 60 litre tank will last you the full      lifespan of two goldfish and will be easier to maintain.  The larger the volume of water, the more stable the water chemistry is and the easier it will be for you to keep your water clean and healthy—it will save you money and time in the long-run!

Do I need a filter?

YES!  The filter circulates (and oxygenates) the water, physically removing fish waste and uneaten food by drawing it through a sponge or fine wool pad.  “Good bacteria” in the sponges then digest and break down the fish waste into harmless by-products.  Without a filter the water will become cloudy and stagnant, and the fish will become ill.  All our tanks are sold as a kit with a suitable filter.

What gravel should I use?

To some extent this is just a matter of taste but ideally you should choose gravel with a grain size of about 2-3mm as this is the easiest to keep clean and grow healthy plants in.  Finer gravel or sand will compact and cause plant roots to rot/die and will be sucked up your gravel cleaner.  Larger-grade gravel will trap debris and fish waste, which will pollute the water and cause algae to grow.

Do I need to add anything to the water?

Our tap water contains chlorine, chloromines and heavy metals which are harmful to gill and mucus membranes.  Chlorine also kills the beneficial bacteria in the filter, which would stop the filter breaking down fish waste safely.  You need to add a ’water conditioner’ to       neutralise the chlorine, and some ’live bacteria’ to kick-start the bacteria in your filter.

How should I decorate the tank?

Live plants oxygenate the water and provide cover and a natural alternative food source for your fish.  They also feed on the nutrients in the water that would otherwise cause  algae, so more live plants = less algae!   Decorating the aquarium with rocks, wood and/or       ornaments makes the tank more interesting, both for you to look at and for your fish to explore.  Your fish will be more active and confident if they have plenty of hiding places and decor to swim through.

Do I need the light on all the time?

Ideally have the aquarium light on for about 7 hours a day.  You can put it on a timer so it’s on for a few hours in the morning and then on again in the evening (adjust the timing to fit with your routine so it’s lit for you to enjoy when you are home).  Too much light can cause algae growth (so make sure your aquarium isn’t in front of a window or in direct sunlight!).

How do I make sure the water is healthy?

You will need some test kits so you can monitor the water quality.  The first thing to test  is the pH level (how acidic or alkaline the water is).  Most coldwater fish will be happy with a pH of 7—7.5 which is neutral to slightly alkaline.  This can easily be maintained by    adding a pH buffer (a simple powder treatment) to the aquarium.

It is also important to keep an eye on the nitrite level.  Nitrite is a by-product of           decomposing fish waste and is highly toxic to all fish.  The nitrite level can increase when you add new fish and after you clean the filter sponges (because there isn’t enough good bacteria to break the waste down quickly).  To keep the nitrite level at 0 you need to be careful not to add too many fish too quickly (roughly 2 weeks between each addition is best) and keep adding good bacteria to boost the levels every time you add new fish or do any cleaning.  Nitrite and pH levels are easy for you to monitor at home with test kits and if you are ever concerned about your fish or water quality, just bring us a sample (half a jam jar) of your tank water and we’ll double-check it for you.

What do they eat?

Most coldwater fish are happy with a diet of good-quality pellets, granules or flakes.  The really important thing is portion-control…. They can eat lots of food in one go but can’t digest it all properly, so the more they eat, the more undigested food they release into the water, and this will affect the quality and clarity of the water.  Two or three tiny meals a day will keep them happy and healthy, and will save you money!

What if algae starts to grow?

Algae is like a simple version of a plant and needs light and nutrients to grow.  If the tank is in direct sunlight or you leave your tank lights on for too long, and/or you overfeed the fish you may find your water turns green or you get green algae growing on the glass or ornaments.  It is not harmful, but doesn’t look very attractive!  Usually if you fix the cause (reduce the lighting and feeding) and scrub it off, it stays under control.  There are    products you can add to treat the algae if you want a quick-fix but relying on these can be expensive in the long-run.  Have plenty of live plants to help compete with the algae.

How do I clean the tank out?

Your filter keeps the tank clean on a day-to-day basis and by using water conditioners, regularly testing the water quality, and being careful not to over-stock or over-feed your fish, you should only need to do a water change roughly every 3-4 weeks.  You simply  remove 25% of the tank water (ideally using a gravel cleaner – ask us for a               demonstration!) and then wash out the filter sponges in the ‘old’ water.  You then top up the tank with fresh water (remembering to add more water conditioner and filter         bacteria).  You should never wash the sponges under the tap as the chlorine will kill all the good  bacteria you’ve worked so hard to build up!


Got questions? Talk to us 🙂             01423 869590        harrogate_aquatic@hotmail.co.uk


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