How to avoid fish mortality in the pond?

To ensure our fish feel comfortable and stay healthy in their pond, providing them with a large space or food is not enough. Indeed, there are four essential points that must also be addressed.

In this article, we will try to detail everything you need to know so that your fish live as long as possible in their pond.

Identify the cause at a glance

Fish behavior and circumstances often provide an initial clue. This table does not replace a water analysis, but it helps to know where to start.

If your fish are breathing at the surface, also read our article on fish gasping at the surface: causes and solutions.

Four essential points to respect for the safety of your fish

1. Pond protection

When you decide to raise fish, you must strictly follow all safety measures for your pond.

The first reason your fish might die is "suicide." By "suicide," we mean that fish can jump out of their aquarium or aquaponics tank. It is therefore crucial that you think about installing protection to prevent this from happening.

2. Oxygenation

The second point to look at is oxygen. Fish breathe just like us and therefore need good oxygenation at all times.

⭐ You may also be interested in: the list of oxygenating plants to keep a pond balanced

You must increase oxygenation if:

• you have a high density of fish.
• the water temperature is high: the problem with warm water is that oxygen dissolves less effectively, meaning fish have less available. The lower the temperature, the more oxygen the water contains for your fish. Therefore, when temperatures rise, it is absolutely necessary to add oxygen to the water.

At night, plants and algae no longer produce oxygen but instead consume it: levels are often at their lowest in the early morning. An air pump with an airstone, a waterfall, or a fountain that agitates the surface helps maintain a good level. To choose the right method, consult our guide on how to oxygenate pond water.

3. Check your pond's temperature

The third point to avoid: having an incorrect temperature unsuitable for your fish. Depending on the species, there are specific temperature ranges, much like for humans. We have a certain tolerance for temperatures, whether cold or hot.

⭐ You may also be interested in: our collection of thermometers for ponds

Obviously, it will depend on the fish you have in your aquarium or tank, or the ones you intend to introduce.

For example:

• Tilapia are warm-water fish: they thrive at 25 to 30 °C, eat less below about 20 °C, stop feeding at around 16 °C and risk dying below 12 °C.
• Trout, however, require cool, well-oxygenated water: above about 20°C, they struggle.
• Goldfish and white goldfish can tolerate a wide range: from very cold winter pond water to about 30°C in summer, provided sudden variations are avoided.

It essentially depends on your species. Check the temperature ranges they tolerate and ensure your water stays within those limits.

4. Check the ammonia level in your pond

Finally, the last point to address is the excess of ammonia in your water. Your fish will die when there is an excess of ammonia, and this happens more quickly if your pH is basic.

⭐ You may also be interested in: the list of oxygenating plants to keep a pond balanced

Reminder on the nitrogen cycle

You have your aquarium, your fish. Fish produce waste. Excess food also creates waste that turns into ammonia—which is highly toxic to fish—and ammonium, an ionized molecule.

This entire transformation depends on the pH: if the pH is neutral or below (i.e., 7), we will only have ammonium. Roughly speaking, there will be 1% ammonia, which is not very dangerous for our fish.

If, however, this production occurs at a pH of 8, we will have about 5 to 10% toxic ammonia (depending on the temperature) and the rest as less toxic ammonium. This is already getting worse for our fish. It is between 5 and 10 times more toxic than at a pH of 7.

Finally, if this chemical transformation occurs in water with a pH of 9, ammonia will represent between 30 and 50% of this production, and ammonium between 50 and 70%. Overall, if we take the highest values, between a pH of 7 and a pH of 9, you will have 50 times more toxic substances—ammonia—in the water for your fish.

It is therefore very important to control and regularly check the pH in your aquariums and aquaponics tanks, or risk having high levels of ammonia that will kill your fish.

To know where you stand, a complete analysis is better than checking a single parameter: ammonium, nitrites, pH, and carbonate hardness (KH) should be evaluated together. Our article on how to perform a water analysis for my pond explains the procedure.

What to do in case of mortality?

When one or more fish die without an obvious cause, act in this order to protect the remaining ones:

1. Remove dead fish without delay: their decomposition loads the water with ammonia.
2. Aerate as much as possible: air pump, fountain, waterfall. It is risk-free and often decisive.
3. Stop feeding for one or two days to reduce waste.
4. Analyze the water: ammonium, nitrites, pH, KH, and temperature.
5. Perform a partial water change if ammonium or nitrites are high, using water of a similar temperature, preferably in small repeated quantities rather than all at once.
6. Observe survivors: if signs of disease appear (spots, damaged fins, rubbing), seek advice before treating.

Fish mortality risks by season

Fish do not die for the same reasons in July and in January. Knowing which danger lurks in each season lets you act before the first losses, and target the right checks when a fish is found dead.

Fish deaths in winter: the right reflexes

When the water cools, fish slow down, eat less and less and stay near the bottom, where the water remains the most temperate. It is not the cold itself that causes the most problems, but what happens under a closed layer of ice: gases from decomposing leaves and sludge can no longer escape, and oxygen no longer enters the water.

• Keep an opening in the ice with an ice-free system or a pond heater.
• Never break the ice by hitting it : the shocks travel through the water and seriously stress resting fish.
• Reduce, then stop feeding when the water gets cold: digestion slows sharply and uneaten food rots on the bottom. See should you keep feeding your fish in winter.
• Remove dead leaves before winter : less rotting material means less gas under the ice.
• Place the air diffuser near the surface, not on the bottom: stirring the whole water column cools the zone where the fish spend the winter.

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For the full list of seasonal tasks, read the right steps for your pond in winter.

My fish are disappearing from the pond: what is going on?

Fish going missing without a body being found is rarely a disease. The most common causes:

• The heron : it fishes early in the morning or late in the day, often unseen. A shallow pond with open banks is an easy target. Our advice to scare off a heron and keep herons away from your pond for good.
• Cats and other visitors : they mainly attack small fish swimming near the edge.
• Hidden fish : in cold weather or after stress, fish take refuge in plants or on the bottom and can stay out of sight for several days.
• A dead fish not found : it often sinks to the bottom before rising again. Check the water if other fish seem unwell.

Summary

What are the three most frequent causes of mortality for our fish?

• The first is the fish jumping out of the water. Therefore, put a small protective cover over your aquarium or pond.
• The second is that your fish are lacking oxygen. If there is a high density or if the temperature is hot, don't forget that you will need to adjust the water temperature according to your fish species.
• And finally, carefully check the pH of your water, because if it is too high, the ammonia will kill your fish.

Prevention routine

• Adapt the number of fish to the pond volume: see the quantity of fish to respect in a pond.
• Run aeration continuously in summer and during stormy weather.
• Only feed what the fish can eat in a few minutes.
• Check water quality regularly, and systematically after adding fish or during high heat.
• Maintain the filtration system without cleaning everything at once, to preserve beneficial bacteria.
• Protect the pond with a net if fish are jumping or if a heron is prowling.

For an overview, read our guide on the 4 pillars of a balanced pond.

Frequently asked questions

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