What Are The Brown Spots In Fish Tank? (How To Fix)

brown spots in fish tank

When starting your aquarium-keeping hobby, you might be caught off guard when strange brown spots start appearing inside your fish tank. You might wonder whether your fish are in danger or not, or if your water is dirty, or you might have some other questions. Well, no need to worry; we will provide you with the answers you require regarding these peculiar brown spots in your beautiful fish tank.

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What Exactly Are Those Spots?

These brown spots are, in fact, brown algae. It is a slimy substance that can even take over your aquarium if left unchecked. However, a fun fact is that these brown algae aren’t exactly algae. The removal methods for basic algae will not work on it. This is because these are small animals named diatoms. These brown spots consist of billions of tiny interlocked diatoms. So, in reality, it is actually a bacterial colony.

Why Are These Spots in My Tank?

There are multiple reasons why these diatoms or brown algae appear in your tank. Firstly, this is inevitable because as every new tank starts to mature, these brown algae spots reappear. However, they will dissipate in a few weeks. But there are also other times where it decides to pop up.

Poor Lighting

Low light levels lead to the growth of brown algae. Although this topic is a bit controversial, it is better to be safe than sorry. Consider providing proper lighting to your fish tank. Whether this works or not is another topic for debate, but this solution has worked in some cases.


The presence of silicate in your fish tank water can be the number one reason for brown algae growth. It is the diatom’s favorite food, and they thrive on eating this stuff. You can find silicate in multiple sources such as salt mix, live rocks or sand, and tap water.


Phosphate can also become a factor in the spawning of brown algae spots in your tank. Decaying food, fish, waste, and other items breaking down in your fish tank aid brown algae bloom.


If you don’t find silicates in the fish tank, brown algae can also feed on nitrates, which you can found in tap water, fish food, fish waste, and plant fertilizers.

Ways To Remove Brown Spots

Use Live Corals and Plants

If you add live plants and coral, they will compete for light and nutrients with the diatoms, which would reduce the bloom of brown algae.

Fix Light Levels

You need to ensure that your tank gets the proper lighting and isn’t engulfed in low light levels for too long. It’s better to have more consistent lighting to reduce the spawn of brown algae.

Add Algae-Eating Fish

Certain fish such as blennies, parrotfish, and tangs will consume algae or diatoms and keep your tank free from any brown spots.

Limiting Required Nutrients

You should control the levels of nitrogen, potassium, phosphorus, and silicate in the fish tank. You should keep things such as fish decay, waste, food decay, etc., under control to limit brown algae or diatoms’ growth.


All in all, don’t be worried about the brown spots in your aquarium. Hopefully, now you know the proper ways to take care of that problem and keep your tank clean and spotless. Just be sure not to add anything in the tank that has a negative impact on your fish’s lives and health.

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