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16.2.6. Chemical Algae Control

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Author : David Bogert

Published :

Time To Read :
5 minutes
Difficulty : Level 7

Excerpt :

We do not recommend chemicals to treat algae. The two photos above were posted to an aquarium forum in Facebook. The show what API Algaefix did to a tank. It killed every fish. This is VERY common. Any chemicals (save antibiotics for “blue green algae”) which kill algae are “disinfectants” that will kill ANY cell ... Read more

We do not recommend chemicals to treat algae. The two photos above were posted to an aquarium forum in Facebook. The show what API Algaefix did to a tank. It killed every fish. This is VERY common.

Any chemicals (save antibiotics for “blue green algae”) which kill algae are “disinfectants” that will kill ANY cell they can gain access to. These chemicals “work well” only because the cells of algae do not have shells of cellulose around them like the cells of vascular plants. So all these chemicals kill slightly more algae cells than they kill vascular plant cells.

There are two problems with this approach:

  • The cells of the gills of any fish or shrimp in the aquarium are more easily killed than algae cells are killed. And you do not want to be killing cells in the gills of the fish.
  • The margin between killing a small number of vascular plant cells and killing the vascular plant is very narrow. If conditions are right, one will kill everything in the aquarium with these chemicals.

And, in any case, the copper, API Algaefix, Tetra Algae control, Top Fin Algae Remover, Fritz Algae Clean Out, Flourish Excel™, hydrogen peroxide and the antibiotics are only temporary reprieves. They only kill the algae temporarily. If the underlying problem isn’t solved the algae comes back in a week or two. So I have no use for chemical control of algae. Specifically:

Polyquaternium WSCP

API Algaefix, Tetra Algae control, Top Fin Algae Remover and Fritz Algae Clean Out are all the chemical Polyquaternium WSCP (Otherwise known as Poly(Oxyethylene) (Dimethyliminio) Ethylene (Dimethyliminio) Ethylene Dichloride). We normally avoid anecdotal evidence but this is a case where some exceptions should be made. A comment by one Roy Ware is pertinent:

Argh! I’m an evil (ex) fish owner! I used Top Fin Algae Remover to treat my tank and must of miscalculated the dosage because the entire tank died. Even the snails. It is dead water.

Roy Ware

Here is an interesting write-up on Tetra Algae Control:

Tetra Algae Control
Staghorn Algae
Staghorn Algae

And a write up on API Algaefix:

API Algaefix

And here is another comment on API Algaefix (August 18, 2021):

API Algaefix killed my Rasboras! I’ve heard some other horror stories too! Beware! I’m having a real problem with algae in my 20 gallon so we got this. Used it for 2 days and woke up this morning to 4 of my 6 Rasboras dead! Ugh I’m bummed…Before anybody says read the directions…We did exactly what the directions say except it says use for 3 consecutive days and only used it for 2…

Still another comment (May 16, 2022) on Algaefix:

Algaefix killed all my guppies. I carefully used the proper dose.

Still another comment:·

Just a quick psa to not buy API Algaefix. I had a tank full of African cichlids and a red tail Shark and now there All DEAD.

“Cichlid Addicts”, Caleb Kozak (May 26, 2022):

So Tetra Algae Control and API Algaefix have killed tanks even when used at the proper dosage. This is a very good reason not to use ANY of these products.

Mixed Types of Algae
Mixed Types of Algae

Flourish Excel™

Some recommend using gluteraldehyde (Flourish Excel™) to control algae. Gluteraldehyde is very closely related to formaldehyde and is a disinfectant with no place in the aquarium. This chemical may kill algae, but it will also kill a lot of plants and burn and severely irritate fish’s gills if the dose is even slightly over the recommended dosage. Specifically, there have been several planted aquarium enthusiasts on social media reporting they’ve killed all the plants in their aquarium with Gluteraldehyde. Another social media quote is apropos:

HELP! A total of 7 of my fish died after I dosed Flourish Excel

Excel is gluteraldehyde. Gluteraldehyde kills fish.

Mixed Algae Types
Mixed Algae Types

Hydrogen Peroxide

Hydrogen peroxide is NOT harmless to fish. The hydroxyl radicals produced by hydrogen peroxide will burn the gills of the fish and can kill them. Hydrogen peroxide is just another disinfectant that will kill any cell it can gain access to. These cells can be algae, vascular plants or the gills of fish.


The only exception to the rule that chemicals can kill fish are antibiotics. Antibiotics are harmless to fish. But unfortunately antibiotics only kill “blue-green algae” since this “algae” is actually a bacteria. And antibiotics can subtly harm fish by killing the beneficial bacteria in the filter. This can produce ammonia and nitrite spikes and damage the fish.

"Blackbeard Algae"
“Blackbeard Algae”

Recommended Regimen

Now if one is absolutely convinced chemicals are the way to go use the following regimen:

  • Leave the filter going to ensure flow, but remove the filter media temporarily.
  • Dose aquarium with 3% hydrogen peroxide treatment (1.5 ml/gallon), wait for an hour
  • Immediately do a 50% water change
  • Right after water change add filter media back in and dose with Seachem Excel (follow dosage on back of bottle).

The hydrogen peroxide weakens the algae so when you hit it with the Excel it doesn’t survive. But I warn you, don’t be surprised if you lose some fish and some plants.

Note that many breeders just allow the algae to grow in the aquarium as part of the complex montage known as periphyton. It feeds small fry. And a green aquarium will normally resolve itself in a few weeks if one is just patient and leaves the aquarium with no water changes and the light on 24/7. Using Mother Nature to resolve algae problems is much better than using chemicals.

Green Water Aquarium
Green Water Aquarium

More Information on Controlling Algae

There is a lot more information on how to control algae in these links:

16.2. Controlling Algae in the Aquarium

16.2.1. Controlling Algae in the Fish Only Aquarium

16.2.2. Controlling Algae in the “High-Tech” Planted Aquarium

16.2.3. Controlling Algae in the “Low-Tech” Planted Aquarium

16.2.4. Algae Eaters

16.2.5. The Algae “War”