Winter Pond Care

Winter pond care

By Dave Hulse, Technical Consultant at Tetra


As the autumn evenings start drawing in and the temperatures drop we can see the natural world preparing for winter. Animals make the most of a bounteous supply of fruits and nuts to stock up before winter, annual plants die back relying on their seeds to continue the generations and the leaves on deciduous trees turn brown and fall. A similarly profound change is occurring in the garden pond though it may not be immediately obvious to the pond keeper.

As the air temperature declines, so too does the water temperature, but not in such an erratic fashion. Frosty autumn mornings can often turn into warm afternoons and the air temperature may fluctuate by 15⁰C. Water has a very high ‘specific heat capacity’, meaning it takes a lot of energy to change its temperature. This means large daily fluctuations in temperature will not arise, but the overall effect is that the water temperature slowly declines. This declination, along with the reduction in day length and sunlight intensity will all be signals to prepare for winter. 

What foods should I offer my fish over winter?

Fish are ectothermic. This means their body temperature conforms to the environmental temperature and so their body temperature will be equal to the temperature of the water around them. (The prefix ecto refers to ‘outside’ the body). Every 10⁰C drop in water temperature will lead to a halving of the metabolic rate of the fish. Therefore, not only will the fish eat less and require less oxygen, as the water is cooler, the fish will also be much more sluggish and therefore more vulnerable to predators.

Research is inconclusive about a threshold temperature below which carp will not feed, but a point anywhere between 4 to 12⁰C is likely. However, reports of young carp feeding at temperatures down to 0.5⁰C exist1!

Offering pond fish food in winter is vital, but we cannot expect them to eat their typical summer feed.  During the colder months, Tetra Wheatgerm Sticks are preferred as wheatgerm offers the fish a much more digestible protein, which is very useful to them in the winter months. With this in mind, it’s always best to switch to wheatgerm foods when water temperatures drop below 10°C and continue to offer a small amount of food down to temperatures when the fish naturally cease feeding.

Many pondkeepers believe the fish cease feeding altogether over winter, which is certainly not the case in the wild. Those reports of farmed carp eating pelleted feed at <4⁰C also report the presence of copeopods and bloodworm in the gut of the fish. In captivity if the fish are denied food over the winter then they have to resort to plundering fat and muscle reserves of energy which means they enter spring in poor condition, potentially more prone to pathogen infection.

Should pumps & filters be left on over winter?

It is vital that waterfalls, fountains, air pumps or any other aeration devices are switched off in winter and there are several key reasons for this. Firstly, cold water can hold much more oxygen than warmer water, so we don’t need to aerate as much. Also, as we have seen above, the fish’s requirement for oxygen is much less as the water is so much cooler in winter. So, more oxygen is available, but the fish do not need as much. Much more importantly though, aeration increases the contact between pond water and icy cold winter air which super chills the pond water and this can cause levels to plummet to where the fish suffer.

Therefore, in winter switch off fountains and air pumps. Filter returns via waterfalls should be diverted so that flow returns under the water surface causing minimal disturbance. Raising the pump 1-2 ft from the pond bottom encourages a layer of mildly warmer water to remain undisturbed at the pond bottom where the fish can retreat. Gravity fed systems should return under the water level of the pond. Try to reduce flow rates through the filter, and perhaps feed the filter via a single bottom drain rather than multiple feeds. Any exposed pipework should be lagged with bubble-wrap or similar to help minimise chilling of the water. Ultraviolet clarifiers should also be taken indoors or lagged as they are susceptible to frost damage.

Nitrifying filter bacteria will see a huge reduction in their populations, due largely to the decline in temperature, but also the reduction in the input of nitrogenous waste in the pond from reduced feeding. These bacteria are unable to form dormant stages but can survive for many weeks in the absence of a food substrate. The filter sponges should be kept clean but the flow of water through the filter can be substantially reduced without any problems.


Should I break the ice on the surface of the pond?

The formation of an ice sheet over the surface of the water can prevent oxygen from getting into the pond and can stop carbon dioxide escaping. If an ice layer forms over your pond, it is vital that an ice hole is opened up to allow an exchange of gases between water and air. Do not however, simply smash a hole in the ice with a hammer; reports of fish ‘deafened’ by shockwaves created from breaking ice over a pond are likely to be massively exaggerated. However, we should remember that fish have very sensitive hearing and pressure detectors down their lateral line. A smash to the ice layer is going to send a large pressure wave through the water which will be very stressful and alarming to the fish in the pond. Be kind to the fish and create a hole in the ice by placing a saucepan of boiling water on it. Ice holes can be maintained using a floating pond heater if winter air temperatures are persistently very low.

Winter is a time of minimal activity in the pond, but that doesn’t mean you don’t have a role. Feeding the fish a winter appropriate food, such as Tetra’s Wheatgerm nutrition, is essential to ensure good condition for the spring, while the hardware needs to be managed to ensure minimal aeration and protection of vital components from frost.


1Bauer, C. and Schlott, G. (2004), Overwintering of farmed common carp in the ponds of a central Europe  

Controlling Nitrates

Controlling nitrates in your aquarium

By Dave Hulse, Technical Consultant at Tetra


High nitrate levels are a problem all fishkeepers will encounter at some point, whether that’s dealing with the problem directly or treating one of the biggest knock-on effects of high nitrates – algae! Nitrate and phosphate levels are normally quite low in most aquariums, helping to keep the growth of algae at bay. However, if conditions rise then algae will take advantage and their population will bloom, which can make your tank look really unattractive.

Traditionally we thought of nitrate as non-toxic, certainly to adult freshwater fish, and certainly at concentrations likely to be encountered in the aquarium or fish farm. This assumption is starting to be questioned as our tools for researching fish physiology, behaviour and overall welfare become more advanced.

 Where does nitrate come from?

When thinking about nitrate control in the aquarium it helps to think first of the inputs and the outputs. There are two sources of nitrates in the aquarium, the waste from your fish and the ‘ambient’ nitrate level - the concentration in your tapwater. Fishkeepers living in upland rural areas will probably be lucky enough to have concentrations of nitrate in their tapwater so low that they would be undetectable with standard aquarium testing kits. However, many parts of the country are described as ‘nitrate vulnerable zones’ where the inputs of nitrogenous fertilisers and animal manure on the land is controlled to prevent further elevation of an already high nitrate level in the water supply.

In the aquarium, nitrate from fish and plant wastes will be primarily effected by the quality of the fish food. A typical tropical fish flake food, such as TetraMin, may have a protein concentration of around 45%. Typically, we would expect 16% of this protein to be nitrogen, and therefore for every 1 gram of food added there could be up to 72 mg of nitrogen released which should be rapidly cycled from ammonia to nitrate assuming there is a healthy biofilter. This is where food quality becomes important. Protein is used by the fish for growth and the repair or tissues, but also for energy production in the absence of sufficient oil in the diet. A poorly formulated diet will fail to spare protein for growth leading to higher ammonia production. Using poorly digested protein sources will lead to more protein in the faeces which is destined to be broken down to ammonia and on into nitrate.

Another way that nitrate may leave the aquarium is through bacterial denitrification. The nitrogen cycle sees bacteria that live in oxygen-free environments using nitrate as an oxygen source (nitrate is N and 3 oxygens: NO3-). These oxygen free pockets can be found deep in the aquarium gravel. Products such as Tetra EasyBalance rely on these bacteria to lower the nitrate level of the aquarium. In a large scale trial using a range of home aquariums, the average nitrate level dropped from 40 mg/l to below 20 mg/l over a mere 8 weeks. In laboratory trials over a 24-week period, EasyBalance maintained a nitrate level of around 10 mg/l when the level in the untreated control tanks soared above 180 mg/l.

Helping to prevent to outputs

So, how can we properly treat and decrease these outputs to stop the build-up of nitrate in our water? Sticking with the fishes’ faeces first, a tank that is kept clean with a gravel hoover and filter sponge cleaning will remove faeces from the water before it can fill the tank with more nitrate. Plant decay in the water will also release nitrates from the proteins within their tissues. Proper tank hygiene and removal of decaying plants will remove protein before it can become nitrate.

The best way to keep nitrate build-up at bay is via a partial water change. Just think, ‘the solution to pollution is dilution’! However, it is not simply a case of a 20% water change leading to a 20% reduction in the nitrate concentration. We have to also factor the ambient nitrate level in. In many instances, fishkeepers find that their large partial water change is having no effect on the nitrate level according to their testing kit. At high concentrations, it can be quite tricky to see a difference in 75 and 100 mg/l on a colourimetric nitrate testing kit. Couple this with a high ambient nitrate concentration and the actual drop in nitrate concentration following a 25% partial water change with tapwater whose nitrate concentration is 25 mg/l, may be from 110 to 80 mg/l. This is a considerable decline but is hard to actually detect via a simple colourimetric nitrate testing kit.

Ambient nitrate levels in your tap water

The ambient tapwater nitrate level can really hinder nitrate control via water changes in the freshwater aquarium, so one could go down the route of tapwater purification via reverse osmosis or deionisation. However, if we explore other outputs of nitrate from the tank we can recruit simpler, cheaper alternatives. As mentioned above, the principal reason we want to manage the nitrate level in the tank is because, along with phosphate, it is likely to lead to algae problems. All plants need nitrate and phosphate as macronutrients, so why not grow live plants in the aquarium to outcompete the algae? For many fishkeepers, a fully planted tank, with substrates, high intensity lighting and CO2 supplementation is a challenge too far, however there may still be low-light-loving plants that do not require a substrate that could be suited to your tank, such as Java fern.

Holiday Feeding

Taking care of fish while you’re on holiday

By Dave Hulse, Technical Consultant at Tetra


For many across the country, the summer break is a time to switch off from work and put your feet up, but for fishkeepers looking to get away it can be a cause for concern. However, with an understanding of the possible changes our absence might have on an aquarium we can ensure there is a minimal impact to fish and plants while you are away.

One of the main questions fishkeepers have when going on holiday is how fish will be fed. Before you panic, let’s look at fish eating habits in the wild.

Food fasting in the wild

Food abstinence can occur naturally for seasonal or behavioural reasons; wild temperate fishes for example may not eat over the winter. However, this fasting is associated with environmental changes that are vital cues to the fish to adapt its physiology to a period of fasting. Depriving fishes of food is routinely performed by fish farmers and ornamental fish exporters as fasted fishes excrete much less ammonia and have a reduced oxygen demand than those with full bellies. This shows us that fish are able to tolerate periods without food, but for only around a week at a time.

Feeding your fish while you’re away

With a summer holiday, a lack of food is likely to be harmful to our fish, so how can we ensure they are fed while we are away? Two principal solutions present themselves; holiday foods and automatic feeders.

Holiday foods are blocks of low nutrient value food bound to a soluble matrix. It is vital that the food is not rich in proteins and lipids as normal fish foods, as this would seriously pollute the tank. For example, the protein concentration of TetraMin Holiday food is only 3%. Traditional holiday food blocks bound the food to plaster-of-Paris, which then slowly dissolved in the water releasing the food. However, we have since found that this soluble matrix can markedly raise the hardness of the water, possibly to the detriment of some fishes. That’s why, Tetra Holiday foods use a soft gel which the fish can forage on without altering the water quality. For fishkeepers heading away for a shorter break, feeding fish Tetra Weekend food is a great way to make sure they’re getting all of the nutrients they need for up to 6 days.

The most notable point about holiday foods is to allow your fish to get used to the food before you leave. Many animals exhibit neophobia, (fear of ‘new’ things). This is a wise precaution in the wild as a novel object might be food or might be concealing a predator! Put simply, for a majority of aquarium fishes, if a large block of something that smells a bit like food but looks nothing like it suddenly appears in the tank, many fish will shy away – in case it conceals a predator. To overcome this, add a small amount of the holiday food at the same time as you usually feed. Fish will soon habituate to the holiday food and it will no longer be new to them. Fish are a lot cleverer than we give them credit for.

For fish that are likely to gobble up the holiday food in one go we can use an automatic feeder, such as Tetra’s MyFeeder, that will dispense pre-set quantities of food at programmed intervals throughout the day. As with the holiday food, do a trial run before you leave, ensure the food is all eaten within 2 -3 minutes then adjust the amount accordingly.


Ensure that your aquarium is maintained too

When leaving your aquarium for your summer break, there is also the concern that there may be an equipment failure that will not be rectified until our return. A lighting failure is disastrous for plants but should not be the end of the world for the fish, however a clogged impellor leading to filter failure can be more of a problem.

An IP camera can be connected to your home network allowing you to monitor the tank over the internet in your absence. Cloud-connected water quality monitors are widely available also, this is the kind of tech that gets fishkeeping experts like me excited! Just have an action plan in place should a disaster be detected – who is going to go and rescue your fish in your absence?

Asking a friend or neighbour to check in on your fish will give you the peace of mind that everything is OK while you’re away. Just make sure that, if they’re feeding your fish for you, that they know how much and how often to feed – again practice is essential. Dispensing precise amounts of feed into press-seal bags is one way to ensure the correct amount is added by your friend at each feeding.

One thing to remember is that, under the Animal Welfare Act (2006), we as fishkeepers have duty of care over our fishes. Neglecting to feed them or our absence from their habitat leading to us failing to take action if their environment rapidly deteriorates, would be seen as a failure in our duty of care. We would not leave a dog or cat alone for two weeks and the same is the case with fishes, especially in the eyes of the Animal Welfare Act.