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Why Is My Molly Turning Black? (With 4 Easy Solutions)

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As a fish owner, dozens of times, I’ve noticed changes in my fish’s coloration. For example, there were periods when I saw that my mollies gradually turned black. As I studied the phenomenon, I learned how to distinguish an underlying illness from a natural coloration process. To make sure that you take the right action, I decided to share my conclusions.

Mollies tend to become black as they grow. That is primarily manifested in dalmatian mollies, as they feature dark spots during maturity. Nevertheless, mollies also blacken when they are stressed, suffer from elevated ammonia concentrations, or carrying a disease, such as Black Spots and Fin Rot.

As we move forward, I will share all the necessary steps you should take if your molly is turning black. I will also elaborate on the particular case of the dalmatian molly, which naturally becomes darker as it grows. 

Why Is My Molly Turning Black?

Mollies can turn black for many reasons, and it isn’t always a cause for concern. For some mollies, this change is merely cosmetic, that is to say, it doesn’t mean anything where the molly’s health is concerned. 

However, for other fish, this drastic color change is a sign of trouble. You have no way of determining the seriousness of the issue until you identify the potential causes, which can include:

1. Genes (Dalmatian Mollies)

Some mollies are genetically predisposed towards this color change. That is especially true for dalmatian mollies, a variation of the sailfin molly.[1] Some dalmatian mollies are white or dull grey at birth. But as they mature, they develop dark spots. 

Over time, those dark spots may spread until the fish becomes completely black. It is a common occurrence that confuses some aquarists. It also annoys others who purchased their dalmatian mollies because they appreciated the white or dull grey colors.

If you are not sure about your molly type, bear in mind that dalmatian mollies are relatively easy to identify. They are called this way because they look similar to dalmatian dogs. You will usually see black spots that look like ink stains across your molly’s body (as in the picture below). These stains tend to multiply and eventually cover the head and tail as well.

2. Fin Rot (Black Fins)

If your molly’s fins are the only organs turning black, it probably has fin rot. Sometimes, this color change is subtle, only becoming apparent when the fins begin to fray. The bacterial infection will eat away at the molly’s fins, making them shorter and shorter until they are gone.

Fin rot is primarily caused by low water quality, overcrowding, and overfeeding, not to mention poor handling.[2] Sometimes the fins will also feature white dots, which could spread to the body if your molly has the Ich.[3]

3. Elevated Ammonia (Black Patches)

Whenever goldfish turn black, aquarists test for ammonia, and for a good reason. The toxic substance is a danger to fish, burning their gills and compromising their breathing ability.[4] Produced by decaying plants, leftovers, and fish waste, ammonia can burn a goldfish’s skin.

Though, surprisingly, the black patches on their skin are encouraging. They are a sign that the goldfish has started healing. In other words, you cannot see the burns left by ammonia on the skin of your goldfish, only the consequences of the healing process.

Obviously, mollies are not goldfish. However, goldfish are not the only fish whose color can change due to environmental conditions such as ammonia spikes. Color changes in fish are governed by a branched cell called ‘Chromatophore’.[5]

Whether a fish darkens or pales depends on the distribution of the color pigment in the chromatophore. Low water conditions, including an elevation of ammonia, nitrate, and nitrite levels, can cause the color pigment to concentrate in a cell. 

As a result, the fish will become darker. Though, bear in mind that it can also become lighter. Similar consequences can manifest as a result of the wrong pH and water hardness. You typically see this case in a poorly maintained aquarium with a lot of debris and leftovers.

4. Black Spot Disease (Black Bumps)

Also called fluke disease, black spot disease has been traced back to larvae that grow in a bird’s intestines.[6] The larvae eventually reach the water via the droppings of the bird. They mature and infect snails before developing further and making the leap to fish.[7] 

When these parasites settle under the skin of fish, black spots appear. Birds are infected by these parasites when they eat diseased fish. You can introduce black spot disease to your molly fish tank through infected snails.

Therefore, if your tank features a few snails, you should suspect the Black Spots disease. You may raise your suspicions even more if the snails are relatively new to the tank. You will typically see black bumps spread across the infected fish’s skin, gills and fins.[8]

5. Stress (Black Eyes)

You wouldn’t expect stress to cause a reaction as drastic as a change in color, and it doesn’t. Stress doesn’t cause mollies to turn black. However, it can affect their eyes, causing them to darken. Studies that were done on guppies found that their eyes could darken in response to aggression and trauma.[9] Similar results were found in Tilapia.[10]

Scientists don’t know for sure how stress changes eye color. But they have enough evidence on record to prove that it happens. And considering the many similarities between guppies and mollies, it isn’t that inconceivable for your molly’s eyes to darken as a result of stress.

Also Read: Stress In Molly Fish

How To Treat Mollies That Turned Black?

You cannot treat a dalmatian molly that has turned black, not if the change happened naturally due to the creature’s age. But if an external factor instigated the transformation in your molly’s color, you can try the following treatments:

1. Fixing Ammonia Spikes

First, I suggest testing the water to confirm that the ammonia concentration is too high. The most effective way would be using stripes or testing bundles. I personally use the API Aquarium Test Kit (link to Amazon). I love that kit because it lasts forever and is highly cost-effective.

Within five minutes, the API Test Kit will measure your aquarium’s pH, nitrates, nitrites, and ammonia. You also get comprehensive instructions to make your life easier. For a molly fish, the pH should be between 7.5 to 8.5.

Also Read: What Is The Best pH Level For Molly Fish?

If the pH is lower than 7.5 or the ammonia is relatively high, you should identify and then combat the factors elevating its concentration, such as overfeeding and inadequate filtration systems. You should also perform a partial water change along with lowering the pH.

Start by replacing 15-25% of the water weekly. I also recommend vacuuming the substrate to eliminate rotting leftovers and plants. Those could spike the ammonia concentration pretty quickly, especially in small tanks. You may also consider active products such as the Aqueon Ammonia Neutralizer (link to Amazon).

You should also test for chloramine, a combination of chlorine and ammonia. Some places use chloramine to treat tap water. If you get your tank water from the tap, you may use commercial products such as the API TAP Water Conditioner (link to Amazon).

2. Treating Fin Rot

Because poor tank conditions often cause fin rot, you should start by carrying out a water change and improving the tank’s conditions. Once this is done, you can treat the fish with antibiotics. I recommend asking a vet for a useful product. 

You can also add aquarium salt to the tank, which will enhance the healing process. When you do so, make sure that you isolate the infected fish in a hospital tank. Then, add half a tablespoon of aquarium salt to each gallon of water the tank contains. For that, I highly recommend the API Aquarium Salt (link to Amazon).

Also Read: Molly Fish Fin Rot

3. Treating Black Spot Disease

Because black spot disease is typically introduced to aquariums via snails, you can cure a molly fish infected with the disease by removing the snails.[11] Black spot disease is less dangerous than IchIch, and it rarely kills its victims. 

Some people use mild antibiotics to treat the disease, although a molly fish can recover from black spots on its own in the absence of antibiotics. You may also consider isolating healthy fish that haven’t caught the parasite.

Also Read: Black Spots On Molly Fish

4. Mitigating Stress

Even if your mollies don’t have black eyes, eliminating stress will help them overcome ailment conditions affecting their coloration. The easiest way to combat stress in your fish is to keep them in a well-maintained tank. 

Mollies are peaceful fish that should be kept with equally peaceful fish. They need temperatures ranging from 78 to 82 degrees F, a pH of 7.0 to 8.0, and hardness ranging from 10 to 25 DGH.[12] It would help if you also reared them in tanks of at least 20 gallons to avoid overcrowding.

Because of their enthusiastic breeding habits, I recommend that you always keep two to three females in a tank for every male molly fish. That will prevent the males from harassing the females to death. It will also lower the competition over a female.

A molly fish living in a clean tank with the right tank mates and appropriate parameters is less likely to suffer from stress, especially if it has plenty of plants and decorations for hiding spots.

If you have no plants or decorations, make sure that you add a few, preferably high ones. Since mollies typically frequent the middle sections, their hiding spots should be in those areas. It would also be best to avoid aggressive fish such as angelfish or betta fish.

Also Read: Why Is My Molly Shaking?

Can Mollies Change Color?

Yes, mollies can change colors. That typically happens in male molly fish that present dominancy and sit at the top of the hierarchy. Those mollies usually feature more intense colors, compared to females or inferior males.

Mollies may also become more colorful as they age. Usually, young mollies are relatively pale, regardless of their gender. However, that process could potentially reverse if the molly is turning too old and is dying.

A molly fish’s diet will also determine its colors. Mollies that rely only on pellets for nutrition may lose their colors and become white. For that reason, I highly suggest adding brine shrimps or freeze-dried food to their diet.

Personally, I use the Tetra Baby Shrimp Sun Dried Treat (link to Amazon). I started using that supplement when I saw that my guppies gradually turned white. Including baby shrimp in their diet improved their coloration within a couple of weeks.

Also Read: Do Molly Fish Change Color?


If your molly is turning black, the first step you should take is to check its type. If you’re growing a dalmatian molly, it is probably becoming darker due to its genes. Dalmatian mollies will naturally feature black spots as they age.

However, if the black areas are primarily in the fins, it could be that your fish is suffering from fin rot. Another prevalent disease is black spots, in which the black areas will also be bumped. Those cases require you to act.

Start by isolating the sick fish in a separate tank. You should also remove any snails that can potentially carry a parasite. Then, consult a vet and start treating your fish with antibiotics if necessary. You should also test your water for ammonia. If elevated, I suggest cleaning the tank more frequently.