Flourish Excel Dosing – Right Way to Dose Liquid Carbon to Planted Tanks

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Flourish Excel is one of the best liquid carbons out there. It is made by the well-known brand in the aquarium hobby: Seachem. Flourish Excel cannot replace a traditional CO2 system. However, if it is dosed correctly, can be really effective growing aquarium plants.

In this article, I will share my experience and test results with Flourish Excel dosing. And, will also talk about the common problems while using this product.

To avoid any damages to your livestock, you should always follow instructions when dosing Flourish Excel. While this product contains very strong compounds (Glutaraldehyde), it can easily harm plants. DO NOT OVERDOSE!

That said, let’s see when and how to dose Flourish Excel to have the greatest success (check current price of Flourish Excel on Amazon).

Please note that the information bellow, are based on my personal experience. You might not get the same results, so please be very careful if you want to replicate it.

When to Dose Flourish Excel?

Although, the manufacturer of Flourish Excel says, that you can dose it anytime, I recommend adding it to your aquarium right after the lights came on.

Flourish Excel is used by the plants during photosynthesis. The plants photosynthesize when they have light and nutrients. In this time, the plants will absorb CO2 through their leaves and will release oxygen.

When lights are off, plans will consume oxygen and will release CO2. In this period, plants are unable to absorb Flourish Excel, so it is mostly wasted.

So, if you want to have the best results, dose Flourish Excel when the lights turn on.

How Often to Dose Flourish Excel?

Flourish Excel will break down in the water in about 24 hours. This means, that after 24 hours it will no longer be effective.

There are many people who dose Flourish Excel every other day, or 3 times a week. From my experience, this is not a good idea.

I got the best results with Flourish Excel, dosing it every day.

Dosing it every other day can cause a black beard algae bloom. This algae type will grow in aquariums where there is CO2 fluctuation. So, to avoid any BBA outbreak, I strongly recommend using Flourish Excel on daily basis.

How Much Flourish Excel to Use?

As I mentioned above, you should always follow instructions from the label, when dosing Flourish Excel to your aquarium.

The label says that you should dose 5 ml (capful) of Flourish Excel to 200 liter (53 gallons) of water volume on daily basis. After water changes, you can use 5 ml of Flourish Excel to 40 liter (11 gallon) of water volume.

I followed the label for about 2 months. It did not work for me. My plants did not grow so well, but algae thrive. Black beard algae started to take over the aquarium.

So, I decided to take some risks and started raising the amount of Flourish Excel.

I’m using Flourish Excel in a 240 liter (64 gallons) aquarium. The water volume is obviously less tank 240 liter, because I have a thick substrate and lava rocks. This aquarium has both slow and fast growing plants. The lights are on for 8 hours a day. I dose liquid fertilizers (both micro and macro) on daily basis. Once a week, I add iron. The tank is not heavily stocked (angelfish, rummy nose tetras, serpae tetras, platties, mollies, swordtails, corydoras and one bristlenose pleco) and I feed once a day.

Here is how I use Flourish Excel in this aquarium:

  • Day 1-2: 5 ml
  • Day 3-4: 7 ml
  • Day 5-6: 10 ml
  • Day 7-10: 12 ml
  • Day 11-15: 15 ml

I started with the recommended dose. Then I slowly increased the dose to not shock the plants.

After a week, I was dosing more than double the recommended amount.

The plans started to grow really good. Slow growing plants such as Anubias and Java Fern started to shoot 3-4 new leaves instantly. Swords and Crypts also started to gain power.

Fast growing plants such as water wisteria, bacopa, guppy grass and waterweed (Elodea) started to gain 1 inch per day in size. I was amazed!

After 12 days, algae started to die off. At this point I decided to not increase dosing, but keep it at the current level. So, I ended up dosing 15 ml / day of Flourish Excel.

I know, this is the opposite of what I stated above: to not overdose Flourish Excel.

Overdosing of Flourish Excel can harm livestock, so it is risky. You need to know what you are doing when overdosing it.

Things to Consider When Dosing Flourish Excel

When dosing Flourish Excel, you need to take in consideration few things:

  • Water volume of your tank
  • Types and amount of plants
  • Aquarium lighting
  • Fertilization of plants

Water Volume

When using Flourish Excel, knowing the exact volume of the water is essential. It is very important especially for smaller tanks.

For a 20 liter (5 gallons) aquarium 0.5ml is the recommended amount of Flourish Excel.

I highly recommend using a syringing for exact measurements. For smaller tanks it is better to start with a smaller dose than the recommended and slowly increase every day, until you reach your goal.

The ecosystem of small tanks is very fragile. So, even a small amount of Flourish Excel can cause significant changes in the tank.

Types & Amount of Plants

Although it is important to know the exact volume of water, in my opinion, the most important are the types and amount of plants. If you have lots of plants in your aquarium, they require lots of CO2.

If you have fast growing plants, most likely, the amount of Flourish Excel, which is recommended on the label of the product, will not be enough for a heavily planted tank.

If you have only slow growing plants, the recommended dose might be too much, because slow growing plants absorbed CO2 much slower.

For aquariums with just few plants, the recommended dose might kill all the plants. It did happen to me.

Aquarium Lighting

All plants require light in order to photosynthesize and grow. Most plants will photosynthesize for about 8-10 hours a day.

So, keeping the lights of your aquarium on for more than 10 hours daily will result in algae bloom. Even though you are dosing Flourish Excel, algae will still grow, because they will use the extra light and nutrients, when your plants are unable to use it anymore.

If your aquarium is placed near a window, and receives sunlight, the lights of the aquarium should be on just for few hours.

Plants Fertilization

Beside CO2, plants need nutrients too. You can fertilize your plants with root tabs and liquid fertilizers. Only use fertilizers, which are safe for aquariums.

If there are not enough nutrients in the water, plants will not be able to absorb the CO2. On top of that, plants will have nutrient deficiency and will start dying off. Decaying plant matter will turn into ammonia and nitrates, which will be used up by algae.

It requires some time, tests and patience to balance out the amount of fertilizers and liquid carbon in your tank. Each setup is different, and has its own complexity.

So, I recommend starting with small amounts first. It is much better to under dose and slowly increase the amount of Flourish Excel and see what works for your setup.

Can Flourish Excel Kill Algae?

The main compound of Flourish Excel (Glutaraldehyde) is used in the agriculture as pesticide and is used to disinfect and sterilize heat-sensitive equipment (source). That said, Flourish Excel can and will kill algae and plants if it is overdosed. If used in very large quantities, Flourish Excel can kill fish too.

So, I repeat myself again: do not overdose!

Killing algae with Flourish Excel will not happen overnight. However, there are few tricks you can use to speed up the process.

You can dose Flourish Excel to your aquarium normally, like you would dose fertilizers or medication. This is the slowest process of killing algae.

Here is a photo of my plants before and after dosing Flourish Excel:


As you can see, almost all the black beard and green beard algae are gone from the plants. I got this result after about 3 weeks of using Flourish Excel. So you need to have patience.

If you want to speed up the process, you can use Flourish Excel for spot treatments. Just measure the amount of Flourish Excel you need into a syringe, and inject it right where the algae grows. Just after 2 days, you will see the algae will start dying off.

You can go even further. If your plants are highly infested with algae, you pull out the affected plants and apply Flourish Excel with a brush. I use this technique to get rid of black beard algae on Anubias plants. You can find out more on this method here.

As mentioned above, you can use Flourish Excel to kill algae. However the goal with this product is not to kill algae. Our goal is to help plants grow faster and outcompete the algae. Eventually the algae will die, because the plants will suck up all the nutrients.

If you want only to kill algae fast, you might want to use a product, which is made for this purpose. I highly recommend the Tetra Algae Control from Amazon.

Flourish Excel Alternatives

Flourish Excel is not very cheap, but is definitely way cheaper than a CO2 system. However, if you are looking for an even cheaper liquid carbon solution for your planted tank, here are some products I recommend:

You might not get the same results with these products as with Flourish Excel, but will definitely help in growing beautiful plants more effectively.

Flourish Excel Dosing for Different Aquarium Size

The manufacturer recommend dosing 5 ml of Flourish Excel to every 200 liter (53 gallons) every day. So, I made the following table with rough estimates of how much Flourish Excel should be added to different size planted aquariums.

Tank Size Flourish Excel
5 Gallon / 20 Liter 0.4 ml
10 Gallon / 40 Liter 0.9 ml
20 Gallon / 80 Liter 1.8 ml
25 Gallon / 95 Liter 2.3 ml
40 Gallon / 150 Liter 3.7 ml
50 Gallon / 190 Liter 4.7 ml
65 Gallon / 250 Liter 6.1 ml
75 Gallon / 285 Liter 7 ml


You might not see results right away when using Flourish Excel, but with time and patience, your aquarium will look fantastic.

I really hope that my article has helped you figure out how to dose Flourish Excel to your aquarium.

Author Image Fabian
I’m Fabian, aquarium fish breeder and founder of this website. I’ve been keeping fish, since I was a kid. On this blog, I share a lot of information about the aquarium hobby and various fish species that I like. Please leave a comment if you have any question.
Questions and Answers

A great article it gives me a lot more to think about My tank is a 75 gal planted tank with about 30-40 fish of all kinds why I am asking is I dose flourish 7 mil 2 times a week and have been adding 7 mill of CO2 booster at the same time in your article you don’t mention the booster very much and I was wondering if fI should cut out the co2 and just stay dosing with flourish I do a i/2 tank water change every 2 weeks and vacuum bottom lightly once a month I just worry that I may be doing damage to fish or plants by using both thanks for any advice john

    Hey John. What is the issue in the tank? You want to fight algae? You want to grow plants? How many plants you have? I can only advice you if I know more about your tank.
    In my opinion you should dose small amount Flourish excel on daily basis. The manufacturer says that it will last for 24 hours in the water.
    If you add Flourish like 2 times a week, you will cause CO2 fluctuation in your tank. This will promote growth of some types of algae, especially black beard algae.
    Please give me some more information and I will try to help.

Danny barrett October 18, 2019 Reply

Thanks for writing about seachem excel. After reading this I can see where I’m going wrong. I’ve been adding seachem excel every other day to a 125 litre tank.i have small amounts of black algae showing.ill def start to add seachem excel daily to my tank.thanks again for the write up.

Hey just a heads up you’ve got the measurements backwards on the dosage. The label say on initial use or after a major water change use 1 capful (5ml) for every 40L (10 gal). Thereafter use 1 capful for ever 200L(50gal).

Great article on a subject that has many opinions based on personnel experiences. I was also not using it on a daily basis and I was not seeing any difference IMHO. Doing research and getting so many mixed opinions on liquid carbon. Might give it a chance again and do further testing. It has been a journey dialing in aquariums for plants 🙂

Will Honeyball April 13, 2020 Reply

Very insightful and useful information, thank you. I was just wondering how often you do water changes on your tank and if you adjust the dosage of excel afterwards?

    I do water changes every week (about 30%). Over some period of time, I found that this works best for my setup.

So I read the label wrong, and have been dosing my 10gal @5ml every other day, and my two 5gal 2.5ml The same. And recently upped that to once a day. Now after reading this, I’m like, ummm now what? My tanks are heavily planted, and grow like I am daddy nature, should I cut it back down, keep at it? I would love it if the plants just stopped growing as pruning is a weekly thing.

    Well, with a planted tank, there will be always a higher level of maintenance, especially if you inject CO2 or add some type of liquid CO2. You can use slow growing plants such as anubias, java fern or amazon sword if you want to reduce the maintenance on your tank.

Ahmet Acar November 5, 2020 Reply

Are you using 3x daily dosage still?
Or did you ignore it after killing bba completely?

    I’ve been using the 3x dosage until I got rid of the BBA. After my plats took off, I reduced the dosage to normal.

      Awesome info! This helped me alot! I use Seachem and wasn’t sure about dosing!

      Thank You!

Is dosing excel 3x the dosage hard on the fish and shrimp?

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