Aquarium Red Slime Algae Causes And Solutions

Aquarium Red Slime Algae - Causes and Solutions

If you have a saltwater aquarium, then you’re likely aware of an algae called Red Slime. It’s classified as a certain bacteria that’s also known as cyanobacteria, and it acts as a byproduct of photosynthesis. This algae most commonly occurs during the beginning processes of setting up your aquarium, so you’ll likely experience them if you’re new to the fishkeeping industry for saltwater aquariums.

It can negatively impact the lives of the organisms and plants that you have in your saltwater tank, so you should remove the algae as soon as you recognize it. But it can be difficult to get rid of the algae if you’re not familiar with it. That’s why I’ve put together a guide that can help you identify algae that’s Red Slime, and I’ve even included some helpful information about how to remove the algae from your saltwater aquarium. Or, our best freshwater plants to entertain your fish. 

What is Red Slime Algae?

little boy hitting his head

Technically, Red Slime isn’t represented in the form of algae at all. Rather, it’s classified as a certain type of bacteria that’s also known as cyanobacteria. It’s an aggressive form of bacteria, and they’re really only found in saltwater aquariums. This bacteria is known to thrive in certain areas of your saltwater fish tank that have too many nutrients in the water and where the overall water quality is poor.

Red Slime bacteria and algae is presented in a reddish-brown color, and it usually starts to grow inside of your aquarium in patches at first. But if you don’t treat the bacteria and algae of Red Slime, then it has the ability to spread throughout your entire aquarium in a thick and flimsy layer.

When you try to remove the algae from your saltwater aquarium, you’ll find that it’s extremely frustrating to try and get rid of. It’s able to grow quicker than other types of algae, and it can grow and spread on practically anything. It can even grow back into your aquarium within a few hours if you don’t remove all of it. Although you may get discouraged when trying to remove it, it is possible to remove the Red Slime bacteria from your tank completely (1).

What Causes it to Grow?

Red Slime bacteria and algae are capable of growing in a wide range of water environments. They grow and spread for a variety of reasons, so you should find out why your saltwater aquarium has algae growing in it. Since Red Slime bacteria and algae are particles that can grow and die off, you should consider what types of things the Red Slime bacteria need to grow and then deprive them of those elements.

Check out our guide on the best reef salt mix

More specifically, Red Slime bacteria and algae require certain lighting that can offer photosynthesis to grow, and they also need nutrients to feed off of in order to grow and thrive. Not only that, but Red Slime bacteria also need a structure of some sort in order to grow. You clearly can’t get rid of every decoration and surface in your tank, since the algae will then just grow on the sides of the glass in your aquarium.

Even though you can’t get rid of the surfaces that the bacteria can grow on, you can provide your aquarium with the proper, healthy, and sanitary water conditions as well as the right lighting conditions in order to prevent the Red Slime bacteria and algae from forming. Other than that, you can examine the main reasons why the algae grows and then figure out what caused it to arrive in your tank to prevent the same circumstance from occurring in the future.

Check out this awesome video on the topic of red slime algae:


Perhaps one of the most common reasons why this type of cyanobacteria appears and thrives in your saltwater aquarium is due to improper lighting. More specifically, having too much lighting in your tank can create the perfect conditions for algae to grow. For instance, having too much lighting in your aquarium can increase the growth and speed up the bacteria’s reproduction processes, resulting in the algae spreading even further than originally.


You should only use lighting sources that contain bulbs that are specifically designed for use in aquariums. Not only that, but you should also make sure that the bulbs have the appropriate total wattage in order to provide your tank with adequate lighting. It’s also important that you leave the lights on for a certain amount of time each day so that the plants and fish in the aquarium are able to experience their conception of “daylight” for the recommended amount of time. Usually leaving them on for between eight or nine hours per day should be enough.

You should also consider the different types of light bulbs that your lighting source has, since each type of bulb is able to release a different amount of light. Not only that, but each aquarium may require a certain type of lighting, so considering what type of bulbs you should purchase is important to ensure that the organisms in your tank are residing in a healthy and sanitary environment.

Excess of Nutrients

Having too high of an amount of nutrients is also known to create the conditions needed in order for Red Slime bacteria and algae to grow and thrive. More specifically, having an excess amount of nutrient like phosphates and nitrates in your saltwater aquarium can cause Red Slime bacteria and algae to appear. Not only that, but having phosphates and nitrates simply present at high levels in the water can encourage the bacteria and algae to grow, since these elements are classified as the main nutrient sources for algae in general.

You can have an excess amount of phosphates and nitrates in the aquarium water due to using aquarium products that contain a high concentration of PO4. So using certain sea salt mixes, activated carbon products, KH buffers, or other sources can result in there being a higher amount of phosphates and nitrates than normal in your saltwater aquarium.


If you have Red Slime bacteria and algae due to having too many nutrients in your saltwater aquarium water, then you should use a RO/DI make-up water that’s been filtered in order to rid the water of the excess nutrients. You may also find it beneficial if you use a high-quality sea-salt mix in your aquarium water.

You may even find it beneficial to add a sponge filter to your existing filtration system, since this can help to remove the excess nutrients from the water. Make sure that the filtration system that you have in general is able to perform both mechanical filtration, biological filtration as well as chemical filtration.

Lack of maintenance

panda relaxing

Simply having poor water conditions inside of your saltwater aquarium can be sufficient enough in order for Red Slime bacteria and algae to appear in your tank. Lack of maintenance seems to refer to not performing enough water changes within a certain amount of time, not having the adequate filtration processes to remove fish waste and other unwanted substances, and it can even include not keeping your water within the right temperature ranges.

Looking for entertainment for your betta fish, toys can do the trick!

Not performing the right amount of water changes can result in excess nutrients and fish waste to buildup, which creates unsanitary conditions. And not providing your tank with the right equipment to maintain the temperature, pH balance, and other chemical balances can also offer negative effects as well as a breeding ground for Red Slime bacteria and algae to grow.


Simply providing your tank with the right equipment is recommended if you have poor water conditions that may result in the appearance and growth of Red Slime bacteria and algae. For instance, make sure you change the water and keep the conditions of the water in the recommended ranges for your fish to live in a healthy environment.

Dissolved Organic Compounds

Having an excess amount of organic compounds that have dissolved in your tank water can also create the breeding grounds for Red Slime bacteria and algae, since it increases the amount of nitrates that are in the water. Having too many nitrates in your aquarium water is problematic because Red Slime bacteria feed on the nitrates, so you’re essentially providing the bacteria with their main food source in order to reproduce.


In order to reduce the amount of Red Slime bacteria and algae from appearing in your tank because of excess nitrates, you should reduce the amount of nitrates in the tank, thus cutting off the bacteria from having a food source. To reduce the amount of nitrates in your water, you should practice sanitary maintenance care in your aquarium, and you should also keep the substrate as well as the other decorations in the tank clean.

You may also benefit from adding a protein skimmer in the aquarium, since it can essentially skim the extra nitrates out of the water. You should also make sure that the live rocks in your tank always have clean surfaces. 

How to Prevent Algae

Clean substrate

One of the most common problems that allow Red Slime bacteria and algae to grow in your tank is having dirty substrate. Substrate is the material that’s located at the bottom of the tank, so practically all of the extra fish waste and other elements fall to the bottom and rest in the substrate. And since the substrate has air pockets in between the particles of the material, the waste and unwanted particles stay there, creating poor water conditions for your tank.

Given that, you should make sure that the substrate remains clean in your tank. And you can do so by vacuuming the substrate often with a siphon or a substrate vacuum. You should vacuum your substrate at least once a month, or during every water change.

Regularly change the water

It’s important that you keep the water conditions in your aquarium clean, so performing water changes regularly is highly recommended in order to offer your aquarium organisms clean water conditions. You should only change about 1/4 to 1/2 of the water in the aquarium at a time, since changing all of the water in the tank at one time can risk imbalances in the chemicals or the pH levels of the tank.

Check our guide on the best aquarium stands

Don’t overfeed

Many people think that putting more food in their aquarium will make their fish gain weight and get chubby. But they aren’t aware of the fact that fish won’t overeat the extra food that you put in their tank. Instead, that fish food will simply sit in the tank’s water and result in it degrading. Because of that, having an excess amount of fish waste and food laying around can contribute to the breeding grounds for Red Slime bacteria and algae to grow, reproduce, and thrive.

From that, you may want to remove any fish food that your fish don’t eat during a feeding, rather than let it sit in the tank.

No overcrowding

Not only should you limit the amount of food that you’re placing in the tank for your fish, but you should also be mindful of the amount of fish that you’re placing in the tank for its size. Not only will your fish most likely become stressed out from having too many other fish around them, but this will also create an over-abundance of fish waste in the aquarium.

By having too much fish waste in your aquarium, your aquarium filter likely won’t be able to remove the waste within the proper amount of time, which results in the waste staying in the water. This allows for unsanitary water conditions as well as an opened opportunity for Red Slime bacteria and algae to grow (5).


Overall, Red Slime bacteria and algae can negatively affect the conditions of your aquarium, since it will compete with your plants and live rocks for the available nutrients in your tank. Even though the algae doesn’t negatively affect the fish in your tank, the algae can degrade the appearance of your tank, making it look unsanitary and unappealing.

Given that, you should remove Red Slime algae from your aquarium if you notice it in the water and on the surfaces of your rocks, plants, or decorations. This guide will help you distinguish what Red Slime bacteria and algae looks like, and I’ve also provided you with the adequate information needed in order to understand how to remove the algae from your tank. It even helps you identity why the algae appeared in your tank in the first place. From that, you can remove and prevent Red Slime Algae from your aquarium after reviewing this guide.

Leave a Comment

Your email address will not be published. Required fields are marked *