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Having fish as pets should be fun and rewarding, and it’s important to know what signs to look for to make sure your fish are healthy and happy.


When fish are sick, it’s not as obvious as when other animals are sick. Often, small changes in how they act can show that something is wrong. A lot of fish diseases are hard to see unless you look closely, but most conditions and diseases make fish act differently.


What are fish supposed to do?

What to look for: A happy, healthy fish usually swims at a normal pace, is aware of its surroundings, and reacts quickly when food is added. Fish that just lie there, maybe on their sides with their fins pressed together, and don’t move when food is put in front of them are clear signs that something is wrong. So, feeding time is a great time to watch for any strange behavior.


What does a fish not usually do?

Parasites can cause fish to act in strange ways, like swimming in short stops and starts, jumping, or “flashing,” which is when fish scrape their sides against a surface. If the fish are hanging near the surface of the pond and seem to be gasping for air, it could be because there isn’t enough oxygen in the water or because their gills are hurt or infected.


Chemistry of Water and Testing

If you notice something is wrong, the first thing you should do is test the water in the pond. Correct water chemistry is important for a healthy environment and is often the main reason why diseases spread in ponds.

In a perfect world, people who take care of fish would test the water at least once a week. Putting the results of the tests in a table or graph would also help figure out how much the water has changed over time. In reality, this doesn’t happen very often. As fish keepers, we usually only test the water when we see something strange. Too often, the results come back with parameters that aren’t ideal, and then something is done to fix the problem. Regular testing would help stop these changes in water conditions, which would keep many fish from getting sick.

The most important things to check for are ammonia, nitrite, and nitrate, which are all part of the nitrogen cycle, as well as pH, KH, and GH (acidity and hardness of the water). The NT Labs Pondlab – 200 Multi-test has tests for all 6 of these in a detailed kit that is easy to use and store.

When setting up a new pond, ammonia and nitrite are the most important things to test for. These will be in the pond when it is fully grown, and even in small amounts, they can hurt the fish’s health. By growing bacteria that turn ammonia and nitrite into nitrate, pond filtration gets rid of these poisons in the water. Nitrate levels will rise over time, so algae blooms should be watched for. If there is ammonia or nitrite in the water, do a partial water change and add a bacterial supplement (NT Labs Pond – Live Filter Bacteria) to the filter. This will quickly make the water less toxic. Look at how much fish food is being added to the pond and maybe stop adding food for a while.


Mature ponds shouldn’t have any ammonia or nitrite in them, but as they age, they will become more acidic. The acidification process will lower the carbonate hardness (KH), which will cause the pH to be unstable, so it’s important to test the water often to make sure the levels don’t drop too low. Pond fish like water where the pH is between 7 and 8, the KH is at least 6 dKH, and the GH is at least 8 dGH. The levels of hardness listed above act as a “buffer” to keep the pH stable. If the hardness levels get too low, the pH can change quickly, which can kill fish quickly. Again, changing the water is a key part of keeping the water chemistry in good shape. Fresh tap water replaces minerals that have been lost in old pond water, which keeps the pH level stable. Always use a dechlorinator (NT Labs Pond – Tap Water Chlorine Remover) to treat new tap water (Mature). This will get rid of chlorines, chloramines, and heavy metals that are bad for you. If the KH or GH levels are too low and fresh tap water doesn’t bring them up to the recommended levels, buffering agents should be added to stabilize the water for the long term (NT Labs Koi Care – GH Minerals Up & Koi Care – KH Buffer Up.)

The fish can be hurt directly and indirectly by the quality of the water. It can weaken a fish’s immune system, making it more likely to get sick from other animals. If you let some water parameters get too high or too low, they can be dangerous. High levels of nitrite, for example, make it hard for the fish to get oxygen into its blood. Even though there is enough oxygen in the water, this can cause fish to gasp at the surface, which is a sign of low oxygen.


Disease Diagnosis

Once the water quality has been checked, diagnosing diseases is important to figure out what the best next step is. Fish diseases can be put into three main groups: those caused by bacteria, fungi, or parasites. There are also viral diseases, like carp pox, that can happen, but they are usually rare and harder to treat when they do.

Symptoms of bacterial infections: Ulcers, fin rot, tail rot, mouth rot, dropsy, and popeye are some of the most common bacterial infections in pond fish. People often think that mouth rot is a fungal disease because it looks like white cotton wool. Dropsy is a late sign of a bacterial infection inside the body. Popeye and red patches on the body are also signs of a bacterial infection. If an ulcer isn’t treated, it can often get a secondary fungal infection, but it should still be treated with an antibacterial first.

Bacterial infections can be treated with NT Labs Pond – Anti-Ulcer, Fin-Rot & Flukes (Bacterad) and NT Labs Koi Care – Acriflavin. This is the pure active ingredient, while the pond treatment has two other compounds in it as well. NT Labs Pond Salt Plus can also help ulcer wounds heal, reduce stress, and give the fish osmotic support while the fish are being treated.


Signs of a fungal infection: White growths that look like cotton wool on the fish’s body or fins are usually caused by an infection in an open wound (e.g. ulcer).

Signs of a parasite infection: The fish has white spots the size of sugar grains on its body or fins, flashes, jumps, gasps at the surface, and looks like it has mucus on its body. Flukes, lice, and leeches are all examples of larger parasites.

Some parasite infections, like Trichodina and Costia, can only be seen under a microscope, but white spot (Ichthyophthirius or “ich”) is easier to spot on the body. When a parasite attaches itself to a fish, it causes irritation that makes the fish flash and jump out of the water.

Infections from fungi and parasites are often treated together because the chemicals used to treat both can be used together if the right amounts are used. Malachite green and formaldehyde are used in both NT Labs Koi Care – FMG Mixture and Pond – Anti-Parasite & Fungus (Eradick). Since F-M-G has a higher amount of formalin, it is only good for koi. If used correctly, NT Labs Anti Parasite & Fungus can be used on all types of pond fish. Both are very good at treating a wide range of infections caused by parasites and fungi.

Other parasitic infections, like flukes, lice, and anchor worms, need different treatments: NT Labs Koi Care – Argusol and Koi Care – Flukasol. Both of these treatments are very good, but make sure you know which parasite the fish has before you use it. NT Labs Koi Care – Permanganate Dip can also be used to treat parasites like lice, leeches, and anchor worms by dipping the fish in it.




How to Fix a Pond

When taking care of a pond, there are a number of important things to keep in mind. Even though it may seem obvious, the most important thing to do before putting medicine in a pond is to read the label carefully. Here are some other things to think about before adding treatments:


  1. Some of the active ingredients in pond treatments are not good for all fish, and this is usually written on the package. Note that if a treatment says it is safe for koi, it is only safe for koi. Do not assume that it is safe for all fish. Most of the time, these koi treatments are stronger and are only made for ponds with koi.
  2. Test the water in the pond before treating it. If the chemistry of the water isn’t right, some treatments can make things worse. Always make sure that the levels of ammonia and nitrite are 0 and that the pH is between 7-8 and the KH level is at least 3-4 dKH (unless using a product to solve an issue relating to one of these parameters.)
  3. Correct dosage: It’s important to have a general idea of how many gallons the pond holds before figuring out how much to use. Some treatments won’t hurt you if you take too much, but others can kill you if you don’t get the right amount. The approximate number of imp. gallons is length x width x depth (in feet) times 6.25. Multiply this number by 4.54 to find liters. Every treatment has a built-in tolerance, so don’t worry if a milliliter or two more is given. Use the NT Labs Dosage Calculator if you’re ever not sure how much to dose your pond.
  4. Don’t mix treatments unless the label says you can. The only treatments that can be used at the same time are NT Labs Koi Care – Malachite and Koi Care – Formaldehyde. Always wait at least 7 days between treatments to make sure there aren’t any negative side effects.
  5. Always treat the pond with plenty of time to spare so you can watch the fish in case they react badly.
  6. During treatment, it’s important to turn off the UV because UV light can break down the medicine. During treatment, most people are told to turn off the UV light for 10 days.
  7. Maintain excellent oxygen levels. Dissolved oxygen in the water can be reduced by treatments, so it’s important to make sure all pumps, waterfalls, and filters are running at their best to keep the water moving and keep the oxygen levels high. During the treatment, this should be done 24 hours a day, 7 days a week.
  8. Safe temperature range: Most treatments work less well below 10°C, and many can only be used between 25°C and 30°C. When temperatures are high, water can’t hold as much oxygen. When you add a medicine to this, the oxygen level could drop very quickly and very low.

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