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Author: Dr. Adrian Lawler  
Title:  Toxic Algae
Summary: Toxic algae 'blooms' can occur in fish tanks (and ponds), and cause distress and death to tank occupants - and can also affect humans. The causes, and how to treat the problem.

Contact for editing purposes:
email: Adrian Lawler <>

Date first published: February 2004
Publication:  Aquarticles (Not previously published).
Reprinted from Aquarticles:
March/April 2004: The Wet Thumb, Cleveland Aquarium Society
Internet publication (club or non-profit web site):

1. Credit author, original publication, and Aquarticles.
2.  Link to  and original website if applicable.
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Printed publication:
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Dr. Adrian Lawler,
P.O. Box 48,
Ocean Springs.
MS 39566
#205 - 5525 West Boulevard
Vancouver, British Columbia
V6M 3W6

Toxic Algae

by Adrian Lawler, Ph.D.
(retired) Aquarium Supervisor (l984-l998) J. L. Scott Aquarium Biloxi, Ms 39530

Toxic algae are generally thought of as organisms similar to red tide dinoflagellates, which can kill numerous fish, crabs, and other species. The west coast of Florida is well documented in having red tide problems. Toxic algae blooms also can occur near floating fish aquaculture cages (where there is a build up of fish excrement and uneaten food under/near the cages), downstream from waste discharges from pig farms, etc. (as along parts of the east coast of the United States), and in other parts of the world for usually too-much-nutrient (usually sewerage) reasons. There are many types of causative organisms of toxic algae blooms; an internet search will provide many references, look under Harmful Algae Blooms (HAB).

The causative organisms are toxic blue green algae, and various species of phytoplankton including toxic dinoflagellates, toxic diatoms, and other types of algae. There are various toxins (brevetoxins, domoic acid, saxitoxins, etc.) involved, including those that produce Amnesic Shellfish Poisoning (ASP), Ciguatera Fish Poisoning (CFP), Diarrhetic Shellfish Poisoning (DSP), Neurotoxic Shellfish Poisoning (NSP), and Paralytic Shellfish Poisoning (PSP) in humans.

A good review of toxic algae blooms plus a table of some of the causative organisms and some of their toxins can be found at:

Another helpful toxic algae site that presents the effects on humans is at:

Human, and other animal, deaths from toxic algae occur each year from contaminated dialysis water, from contaminated drinking water, from eating affected organisms, from eating organisms (filter-feeders) that concentrate the offending algae, etc.

Some aquarists may be unaware, but toxic algae organisms can "bloom" in fish tanks (and ponds) and cause distress and death to tank occupants. Algae blooms, toxic or not, can occur in closed systems when high organics or nutrients are present, can occur in both freshwater or salt water tanks with sufficient lighting and nutrients, and can occur in tanks as small as 10 gallons. Since most fish tanks are aerated well, here we are talking about the algae that release toxins and cause problems, NOT the non-toxic algae blooms that die, decompose, and cause oxygen depletion and subsequent death of fishes due to low oxygen levels in nature. Various species of algae that are toxic release their toxins upon death; these toxins in the water can irritate and cause severe distress or death to tank (pond) occupants that have no way to escape.

High organics or nutrients occur, for example, when:
---substrate, or filters, are not cleaned often enough.
---water changes are not frequent enough, or volume changed is not enough.
---too much food is added to the tank and it breaks down.
---an organism, hiding in the tank, dies and is not noticed, and decays.
---a large crustacean sheds, and the shed is left too long in the tank, and decays.
---new additions are made to a tank, and they are left too long in hopes they will survive (e.g., live rock), and decay.
---a tank occupant shreds living plants into small pieces, and these pieces decay.
---a balanced tank is suddenly changed (go on vacation, turn off tank lights, and algae/plants die and decay-then return, turn on lights, and bloom can occur).
---logs, or other previously living things, are not soaked/cleaned enough prior to using in a tank, and they still have a high rate of decomposition.

When a "bloom" occurs in a tank (pond), the water may appear slightly hazy or cloudy from light scattering off all the numerous small algae in the water. The occupants may show distress (gasping, jerky movements, sudden darting, trying to get out of tank, etc.) and abnormal behavior, or even die. The water should be examined under high power using a dissecting microscope, or using a compound microscope. Numerous small organisms of the same type should be seen (see articles on the different types of phytoplankton to get a general identification).

After the problem has been determined, methods of control should be started quickly. But there is a problem in that more toxin is released on bursting/death of the algae cells, so killing toxic algae will INCREASE toxin levels in the tank. And using a diatomaceous earth (DE) filter to filter out the offending organisms will cause them to die and release more toxin. So one must either add another filter containing enough activated charcoal to filter out the toxins, or put charcoal into the DE filter along with the DE powder. One may have to clean and recharge the charcoal filter several times in order to remove all the toxins from the tank. Discard the DE powder and charcoal when done, and thoroughly wash all filters used.

If, while working near an affected tank, you should feel affected (faint, dizzy, impaired vision, eyes burning, nausea/vomiting, difficulty in breathing, skin and respiratory irritation, etc.) leave immediately and complete tank work using a charcoal respirator or gas mask, making sure you do not breathe any aerosol from the tank that may contain the toxin.

After the fish are no longer affected, then the reason, high organics or nutrients, for the bloom must be attacked:
---Clean substrate or filters.
---Do water changes to reduce nitrates, etc.
---Find, and remove, any decaying extra food, plants, animals, or organic decorations.
---Do not overfeed tank.
---Keep lights on tank for a minimum amount of time.

After the problem has been solved, schedule DE filter use on your tanks on a weekly basis, or keep a DE filter on the tank so there is no explosive build-up of toxic algae again.