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Why do goldfish turn black? causes, how to diagnose, treatment & prevention

Why do goldfish turn black is a common concern especially with inexperienced goldfish pet keepers. Your fish turning black is a possible challenge despite goldfish being hardy and among the easiest fish to take care of.

Why do goldfish turn black

Therefore, the earlier you learn why the better for you before it is your turn to also ask; why is my goldfish turning black?

Mostly, all you need is to be on the lookout for any visible body changes as well as how fast they are happening.

Your fish turning black maybe as a result of various reasons. While some are natural and non-life threatening, some are worrisome and call for urgent attention.

Why do goldfish turn black?

A variety of reasons for your goldfish turning black include:

It’s in the genes:

Most goldfish species are as a result of selective mix breeding. What this therefore means is that the resultant breeds may not entirely hold onto specific qualities exclusively, amongst them, color.

This however is not the case with pure goldfish. Its breeding is specifically meant to produce pure gold colored fish.

Unfortunately, this pure gold colored breed is un-affordable to many fish keeping enthusiasts. As a result, the inferior, mixed low grade breed is what dots many aquariums.

As fate would have it, this could be the case with your pet fish. Therefore, in the course of your pet’s development and growth, genetics will (might) particularly play a role in fish color change.

Notably however, most of the color change is expected in the formative stage and in old age of the fish.    

To blend to its environment:

Like with all living things, different cells make up the skin of fish. It’s from some of these cells that melanin (the black pigmentation) is produced.

As a result, the scales and tail of your fish turning black is naturally from this melanin.

Further, more of the melanin production will be induced by the tank’s lighting condition.

A darker tank (rocks, decorations among others) or one with a dark backdrop will induce production of melanin hence you goldfish turning black naturally.

Scales, fins and tail are most susceptible to this color change although it could still occur on the entire body.  

Black spot disease:

Black spot disease could be another answer as to why do goldfish turn black. Firstly, don’t be too worried because it’s a rare occurrence particularly to mature goldfish. In addition, it’s more prevalent in ponds rather than goldfish aquariums.

Nonetheless, your pet fish is exposed to it if it shares a tank with infected water snails or if birds’ droppings get into the waters.

What’s important to note though is that it’s not an infection but rather a parasite caused condition.

How? Well, the parasites from the snails or birds’ droppings will lay eggs which on turning larvae will burrow under the fish’s skin.

As a result and for protection after the larvae burrows in the skin, it develops hard dark cysts; hence the black spots on your goldfish. Eventually the parasite is released after the cyst bursts.

Notably, the fish can have few staggered black spots or in severe infestation case, large portions of the body turning black

Besides the spotting, the fish feels irritated. You will subsequently notice it brushing its body against the tank’s surface. It will also be flicking a lot.

To eliminate this problem, cleaning of the tank in case of birds droppings as well as removing the snails will do the job. It helps to break the parasite’s life cycle.

However, compared to Ich, the black spot disease is less risky.

Ammonia burns:

This is another of the possible answers to your concern; why do goldfish turn black?

First, ever heard of a fish keeper ask; why is my goldfish turning red? Well, let me explain.

While the two possible reasons here above that could cause black spots on goldfish are natural and not risky, ammonia poisoning is.

Ammonia builds up from the fish waste, uneaten feed remains as well as any matter decaying in the tank.

The toxic ammonia has a corrosive effect not only on the body but also on the internal organs of the fish.

Well, if the water quality situation is not remedied in good time, your pet goldfish skin and scales will suffer ammonia burn. The burns will be showing by goldfish fins and tail turning red.

To improve the water quality, you should immediately do a water change as well as waste/debris removal.  This is besides treating the fish.

However, after treating both the water and the fish, you might notice your goldfish turning black on fins and body. Don’t be too worried. Mostly, they get black patches being the damaged skin and fin part healing.

Your pet fish is now on recovery path from the ammonia burns.

How to manage ammonia spike in your goldfish aquarium:

Ammonia poisoning is the riskiest and perhaps the most lethal among why goldfish turn black causes.

It not only affects the skin but also corrodes the internal organs. This subsequently causes internal bleeding and eventual death if not addressed in good time.

Labored breathing and odd swimming are rather late indicators of presence of ammonia in the tank. This will sadly happen if you are not regular in checking the ammonia levels.

So, what can you do to monitor and manage it in good time?

First, use a good test kit to do the ammonia levels reading.

Though they are somewhat complicated, liquid tests accuracy is higher than that of strip tests.

Thereafter, if ammonia readings are beyond safe levels, then you can do the following;

  1. Depending on the severity of ammonia or nitrite concentration, you should do a 30% – 40% water change. Normally, water changes should be between 15% – 20%.

A 100% change is not recommended (only in extreme unsalvageable cases). This is because you’ll not only unsettle the ecosystem and lose the accumulated beneficial bacteria.

Aquarium cycling takes a while to tank’s maturity and for buildup of the bacteria.

Further, before refilling the tank, you’ll have to de-chlorinate the water or use water conditioners. This is necessary only if you are not using distilled water.

  • Have a good and reliable filtration system that will also help in removing ammonia from the tank.

To the filters, you can add carbon which is highly effective in removing and neutralizing many chemicals in the tank waters.

In addition to the filtration system, there are safe chemicals (say like Seachem Prime) from pet stores that change ammonia to a less toxic form.

  • Finally, the most effective way to avoid and reduce ammonia buildup in your goldfish tank is to avoid overfeeding. Feed only what it can eat in under 2 minutes; food the size of its eye.

Uneaten foods’ decay in the tank is the main source of ammonia. Your goldfish can starve for days and thrive but won’t stand poor water quality consequences.   

NOTE: Most important thing however is that there should be NIL (0 PPM) ammonia and nitrite toxins in the water always.

Will black spots on goldfish go away?

Well, this is dependent on the cause. Naturally/genetically caused black spots of fish are rather permanent. No need to worry about them.

Further, if it’s as a result of black spot disease, then the black spotting will be no more once the snails are removed and the parasites’ life cycle broken.

This is essentially because the black hard cysts will disappear as the fish heals and so will the black spots.

Will ammonia burns on fish go away?

Well, if the damage on the affected body parts was severe, the black smudge on goldfish will unfortunately remain.

They’ll otherwise fade off over time as the fish heals and the water quality improves.


Why do goldfish turn black should now not be a major concern to you having read through this far.

Hopefully, you are now adequately informed of the possible un-natural reasons why goldfish turn black. Watch out for them.

Keep your pet fish happy, healthy and colorful always.

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