Black Mold In Fish Tank | Effective 4-Step Treatment

Dec 5, 2021

Back Mold In Fish Tank

In my decade-long fishkeeping experience, I have had to deal with at least a dozen different kinds of algae. And there’s none I dread as much as the fuzzy black mold in fish tank that proliferates in a blink of an eye and is as stubborn as they come. 

So, if this unwelcome guest has made a home in your tank, you have come to the right place. In this blog, I will detail everything from what causes black mold in fish tank to how to eliminate it and prevent it in the future. 

Let’s begin! 

What Exactly Is Black Mold That Plagues Our Aquariums?

In 99% of cases, the black mold that grows in our tank is the dreaded black beard algae. And interestingly, this fuzzy black mold comes from the family of red algae that mushrooms in high-phosphate environments. Scrubbing it away can help momentarily, but the stuff keeps coming back unless you seek a permanent solution. 

The black molds actually appear at the early stages. However, it takes on a beard-like form when mature, notoriously attaching itself to rocks, decors, and plants. 

What Is The Black Mold’s Texture Like?

I went ahead and touched the black mold, so you don’t have to. Despite the rugged look, this algae is incredibly soft to touch and slippery. It’s almost like handling a patch of dark, wet hair. 

What Causes Black Mold In Fish Tanks?

High phosphate deposits in the water are the number one reason behind black molds in fish tanks. Algae thrive on phosphorus. Another possible cause is worn-out fluorescent bulbs that encourage algae growth instead of demanding plants. And lastly, an unmaintained tank with a high degree of pollution and toxic buildup is susceptible to black molds. 

What Aquariums Are Prone To Developing Black Molds?

Aquariums in areas with old Victorian plumbing are especially prone to developing black molds. In these regions, the water authorities add a relatively high concentration of phosphate in the water to avoid corrosion of potentially harmful lead piping. 

That being said, basically, any aquarium that’s not tended carefully is prone to develop black molds. They can make an unbidden appearance even when we are on top of tank maintenance. 

Are Black Molds Harmful For Fish? 

As unsightly as black molds are, they don’t pose much danger to your fish. They are not known to release toxins in the water. In fact, some fish even prefer hiding or playing around the algae and occasionally nibble on it. 

However, the overgrowth of algae will consume oxygen and block essential sunlight. Also, the disbalance in the tank’s nutrient levels will naturally hurt the fish’s wellbeing.

Therefore, it’s best to nip black mold in the bud before it has any chance of growing. 

Are Black Molds Harmful For Plants?

Black molds often first grow on the leaves and stems of slow-growing plants like java fern and anubias. While the molds don’t release toxins in the water, they will outcompete your plants and hinder their access to nutrients, eventually causing them to die. 

So, once again, it’s imperative to do away with the molds as soon as you see them. 

And that takes us to our next section. Read away! 

How To Remove Black Mold In Fish Tank?

Removing black mold from a fish tank is tricky because it absolutely loves to make reappearances. So, you need to go to its core to tackle it. Minimizing phosphate concentration from the tank is your best bet. Getting rid of solid waste and performing frequent water changes are also pretty effective. And lastly, replace your old bulb. 

Step-By-Step Guide To Remove Black Mold From Fish Tank

In this guide, I have pooled the knowledge from my own experience and those of other hobbyists to present an effective solution. I’m pretty confident it will work. The steps include dipping decors in hydrogen peroxide solution, removing phosphate, inhibiting the mold’s photosynthesis process, and so on. 

Step 1: Wash Your Hands And The Tools You Will Use 

The first thing you should do is wash your hands thoroughly. And don’t forget to rinse the buckets, hose, and other tools if you are going to use them. 

You do not accidentally want to expose the fish to other contaminants that can potentially take their lives. 

Step 2: Remove The Fish And Invertebrates 

Transfer the fish to a second tank. You need to ensure the water parameters in their makeshift home closely match the main tank’s. 

Any abrupt or big changes in the environment will severely stress the fish and potentially shock them. And this can defeat the entire purpose of cleaning. 

Step 3: Clean The Tank Lid 

After removing the lid, place it a little far from the tank and the tank’s content. Next, spray the lid generously with undiluted vinegar and wipe it clean. You can use an old toothbrush, sponge, or paper towel. I prefer using a toothbrush. 

Rinse the lid, apply some more vinegar, and let it sit for about 2 minutes. Then, scrub the lid again and rise with hot water. Rinse it until the vinegar’s scent has vanished. 

Let the lid sit to air-dry while you complete other steps! 

Step 4: Sort Out The Decor 

Take out all the decors, including pebbles and gravel, and place them in a big container. Pour over-the-counter hydrogen peroxide (3%) until everything is submerged. 

Some sensitive plants like anubias and java moss may bear the brunt of the compound’s harsh chemical properties. So, what you can do is place them in a separate container and use hydrogen peroxide and water in the ratio of 1:3.

You can even use bleach for plastic plants in the ratio of 1:20. 

Soak everything for 3-4 minutes and rinse thoroughly with lukewarm water. 

The reason I use peroxide is that it practically leaves no residue. So you can instantly put everything back in the tank if you want. 

However, if you are not keen on using chemicals (I understand), you can submerge everything in boiling hot water in a heat-safe container. 

You will then have to manually scrub away any residue with a toothbrush. Rinse everything with hot water. And repeat the scrubbing and rinsing processes as needed. 

Step 5: Clean The Tank 

You will need to drain the water with an aquarium siphon. Leave 50% in the tank if the infestation is not too strong. If it is, siphon and toss it all out. 

Use the algae scraper to scrape off any mold or algae from the tank’s inner glasses. Also, use an old toothbrush and water to scrub the glass surface. And don’t forget to rinse the tank’s sides with hot water and siphon out the results. 

Next, change the filter cartridge and thoroughly rinse the filter. Add all the decors back in the tank. 

Refill the tank with water as needed. Add dechlorinating treatment and phosphate remover (if needed). Make sure to adhere strictly to the manufacturer’s guidelines, so you don’t end up poisoning the fish. 

The temperature and other parameters in the tank will take at least 24 hours to safely stabilize.

Once the water is clean and safe and the waiting period ends, gently place the fish back into their tank. 

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What Is Phosphate? 

I wouldn’t go into jargon. But, simply put, phosphates (PO4) are the byproduct of practically everything that decays inside a tank. This could include anything from leftover food and feces to decayed plants and algae. 

While fish are affected by phosphate buildup in the tanks, the ones that bear the biggest brunt are invertebrates and corals. By now, you already know that phosphate is the primary food source for many algae types, including black mold. 

And the thing about phosphate is that it’s virtually present in almost all aquariums in the world. However, we may not be aware of it. At lower levels, it’s harmless. But as the phosphate level continuously grows, it will cause an irreparable impact on your fish and plants. 

Phosphate levels in the tank naturally increase as the wastes are broken down. Phosphate comes from both internal and external sources. 

Causes Behind Phosphate Spike In Tank

Let me list a few of them below:

  • Dying algae 
  • Leftover food 
  • Decaying plants 
  • Fish feces 
  • Carbon filter media
  • Tap water 
  • kH buffers 
  • pH buffers 
  • Aquarium salt 
  • Imbalanced nutrition 
  • Wrong lighting 

The list above is self-explanatory, isn’t it? So, I won’t explain all of them in detail for the sake of the blog’s length. However, there are a few points that piqued my interest.

High Phosphorus (PO4) Content In Tap Water 

In areas that still largely rely on old lead plumbing, authorities have to use high phosphate concentrations to avoid lead corrosion, which can lead to dire consequences if left unattended. So, if you see black mold developing in your tank, the first thing you should do is test the phosphate levels. 

Here’s a link to what I use from the API:

If tap water is the likely culprit behind phosphate and black mold growth in the tank, you are left with two options: finding a different water source for water changes or introducing a phosphate-absorbing media. 

You will have to use water distilled or purified using reverse osmosis. This will make sure the black molds starve. However, treating water this way will eliminate all the minerals – both the good and the bad. Therefore, you will have to use a remineralizer to treat the water before adding it to the tank. 

I came across this phosphate-free mineralizer by Seachem while researching for this piece and immediately added it to my cart.  Here’s the link if you’re interested too:

If switching the water type seems like a hassle, you can opt for a phosphate-absorbing media for your filter. However, if you are using this type of media, you need to ensure the filter turns the water in your fish tank at least 5 times per hour. 

A powerful canister filter would be my choice for that. 

Imbalance Of Nutrients 

The tank’s nutrient content imbalance is a lesser recognized but very prevalent reason behind black mold. For example, a lack of sufficient carbon dioxide can fuel the growth of black beard algae. 

In this case, your aquatic plants won’t compete with algae for nutrition due to their weak and inefficient photosynthesis processes. 

Wrong Lighting 

Only a handful of aquatic plants need lighting for over 8 hours. So, make sure that you don’t keep the lights on for more than that. 

When there is a disbalance between the amount of incoming light and the amount your plants actually need, algae makes an entrance. 

What’s The Desired Phosphate Level For Aquariums?

Like I said above, phosphate is present in all tanks. And you can get rid of it 100%. The ideal phosphate level to hinder black mold or any other algae growth is 0.2mg/L. 

How To Lower Phosphate Levels In The Tank?

Credits: Emdot (Creative Commons license)

Maintaining the tank’s phosphate levels is not a one-time gig. You have to continuously work towards it. Making some conscious efforts like feeding sparingly, performing water changes, and adding algae-eating creatures can help you.

Feed Your Fish Sparingly 

Any uneaten food will sink to the tank’s base and serve as a breeding ground for phosphate. Therefore, if you see black molds in the tank, it’s time to reevaluate your feeding style. 

Give small but frequent meals instead of one big meal. And don’t forget to remove any uneaten food right away. 

The kind of food you give also plays a pivotal role. For example, phosphate is used as a preservative in most flake foods. 

However, not all foods are created equal. Therefore, make a habit of skimming through the label before making any purchase. 

Reevaluate Your Water Source 

I’ve already explained this point in detail above. But let me give you a quick recap: Tap water has high phosphate content to avoid lead corrosion in some places. If that’s the case in your area, you may have to switch to distilled or purified water using reverse osmosis. 

However, since these kinds of water are stripped of essential minerals in the process, you will have to remineralize them. 

Perform Weekly Water Changes 

The rule of thumb in the fishkeeping hobby is to perform 20-25% water change every week. Make sure that you use phosphate-free water while doing so. 

And, of course, the extent and frequency of the water change depend upon stocking number and tank size.

Maintain Tank Health 

Maintaining a fish tank is a full-time job. You have to constantly be on your toes to maintain the tank’s cleanliness and health. Just keeping the tank free of debris can go a long way in preventing black molds. 

Make a habit to vacuum the bottom often to do away with decayed plants, fish waste, and uneaten food. 

Clean The Filter Often

Uncleaned filters can quite easily hold enough residue and muck to cause a spike in phosphate levels of the tank. So once again, the rule of thumb is to clean the filter once every month, irrespective of the kind of filter you have. 

However, the frequency is subjective to your filter’s capacity, your tank’s size, and the stocking number. 

Opt For Phosphate-Free Filter Media 

Carbon is an excellent and the most commonly used filter media. However, it can add phosphate to the water. These days, you can quite easily find carbon media that also work as a phosphate absorber. This is a win-win for you. 

On the other hand, saltwater tanks don’t have to worry about this one. Carbon media available for saltwater tanks have been designed to not leach phosphate into the water. Lucky! 

Review The Conditioners 

Most water conditioners, ranging from the ones used to buffer the water to ones used for pH alteration, contain phosphate in them. However, the concentration of phosphorus differs from product to product. 

So, do your fish a favor and research the products well before splurging. 

Add Fast-Growing Floating Plants

Most floating aquarium plants proliferate rapidly and have an enormous need for nutrients present in the water column. Therefore, these plants need a lot of food – and guess what – phosphate is one of their foods. 

Here’s the list of our best floating aquarium plants:

  • Mosaic plant 
  • Water spangles 
  • Mosquito fern
  • Water sprite
  • Red root floater
  • Anacharis
  • Dwarf water lettuce 

Change Your Tank’s Bulbs

As bulbs get older, the light spectrum deteriorates. Thus, plants don’t receive as much light as they need to photosynthesize and grow. However, this light spectrum could be just enough for algae to grow. 

And with no plants to compete with, algae will soon take over the tank. 

Source Your Plants Carefully 

Even the plants and decors that come from big, reputable stores could be the carrier of black moth. Therefore, you should never skip disinfecting them first. 

As paranoid or germophobic as it sounds, you should use hydrogen peroxide and water in the ratio of 1:4 and let the plants and decors soak for a few minutes. Don’t worry – hydrogen peroxide doesn’t leave any residue. 

But still, you can go ahead and clean them thoroughly before placing them in the tank. Better safe than sorry, right? 

Add Algae Eaters In The Tank 

One of the wisest and most effective decisions you could take to get rid of black molds from the tank is introducing algae-eating fish. However, you cannot just pick a couple of random algae eaters. 

Their water needs, dietary patterns, level of aggression, and sleep cycle should be considered first. 

Fish That Will Eat Black Mold/Beard Algae 

Here’s a definitive list of fish and invertebrates that have been reported to consume black mold or beard algae (they are the same):

  • Siamese algae eater 
  • American flagfish 
  • Twig catfish 
  • Rosy barb
  • Cherry barb 
  • Chinese algae eater 
  • Bristlenose pleco 
  • Black molly 
  • Pigmy suckermouth
  • Rubber-lipped pleco
  • Flying foxes
  • Nerite snails 
  • Amano shrimp
  • Goodeids 
  • Emerald crab

I came up with this list after hours of gigging through other blogs and forums alike to find real evidence. But still, we can never be 100% sure who will consume black mold and who won’t. 

More than species, it depends on the fish’s personal preferences. So, good luck with that.

And if you’re among the lucky ones whose fish do eat black molds, remember, you will still need to supplement their diet with other nutritious and more filling food. 

It would be cruel to assume they’d thrive just on algae. 

Black Algae In Fish Tank – How To Get Rid Of It?

Actually, black algae’s full name is black beard algae, and for obvious reasons. It actually grows like a black fuzz that’s quite identical to a human beard. Phosphate is the leading cause behind its appearance and growth in our home aquariums. Therefore, cutting down the phosphate levels will help eliminate the algae. 

As a matter of fact, the black mold that we just talked about above is black beard algae in its most advanced stage! 

As we discussed above, the possible causes behind the spike in the tank’s phosphate levels are overfeeding, feeding food with phosphate, poor lighting, poor water quality, and wrong husbandry practices. 

Switching to RO or distilled water, cutting back on feeding temporarily, performing frequent water changes, and introducing black algae-eating species like siamese algae eater and Amano shrimps can be super helpful. 

Final Words: Black Mold In Fish Tank 

This article got pretty long, didn’t it? I just wanted to be sure that you know everything I know on this topic. So if you didn’t go through the whole article, don’t worry – here’s a quick recap of the essential bits for you. 

Black Mold is actually black beard algae in its early stages that look like black, hairy, and fuzzy patches. 

Although it isn’t incredibly hard to get rid of, it will come back to haunt you again and again if you don’t get to the root of it. 

An increase in the tank’s phosphate levels is the number one reason behind black mold in fish tanks. And phosphate concentration can increase for several reasons like overfeeding, lack of water changes, bad lighting, buffers, and carbon media. 

Performing frequent water changes, using a phosphate remover, feeding phosphate-free food, and switching from tap water to RO water are a few steps you can take to tackle phosphate spike and subsequent black mold growth. 

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rohit gurung author at urbanfishkeeping

About Rohit Gurung

My never-ending love and fascination with Aquascaping started when I received a red-eared turtle for my 10th birthday.

Apart from researching and writing, I spend hours gazing at my 3 turtles. And yeah, I bask alongside them too.