There are some algaecides available that can be used in potable water tanks and some are safe for use in waters that are used for irrigation of edible crops. Certain algaecides are even safe for fish. Always read the label of any product you are using to ensure you are using it correclty and for the purpose it was designed for.
Diquat is a contact herbicide that controls Spirogyra filementous algae.
Diquat is only slightly toxic to fish and if you follow the recommended application rate is followed, you reduce the possibility of harming aquatic life. However, as the aquatic weeds decay, the oxygen level in the pond water begins to deplete. Without the proper oxygen levels, fish may not survive. Using strip applications of diquat will help prevent harm to fish.
Use at a rate of 1 gallon per surface acre (4' deep)
Diquat Dibromide 37.3%. This concentrated Herbicide is for weed/algae control in and around ponds, lakes, commercial greenhouses and nurseries, landscapes, industrial land, recreational land, residential lawns, and public areas.
Another less chemical algae control/cleaning method:
Using a granular algaecide works very well. GreenClean is a non copper based granular algaecide that works quickly to control string algae and other types of pond algae. GreenClean is an algae killer that works on contact to kill algae and help keep ponds clean all year long. GreenClean can be used for algae control in any water feature such as ponds, water gardens, ornamental waterfalls, fountains and birdbaths. It is a non-hazardous algae killer that is safe for fish, birds and pets once added to the water.
It also works on pond algae, especially in conjunction with beneficial bacteria. See more about Green Clean Algaecide
Use GreenClean Granular Algaecide on all listed non-painted surfaces, to prevent and control algae, moss, slime molds and their spores, and the odors and conditions that these organisms cause (sudi as the breeding grounds for pests sudi as shore flies and fungus gnats).
GENERAL TREATMENT NOTES
Control is most easily achieved when growth is not yet well established. Treat when growth first begins to appear.
Apply in a manner that will insure even distribution of GreenClean Granular Algaecide within the treatment area.
GreenClean Granular Algaecide is water activated. Watering before application is preferred over misting after application because it prevents over watering leading to a reduced effectiveness.
Use GreenClean Granular Algaecide in conjunction with ZeroTol Broad Spectrum Algaecide/Fungicide and TerraCyte Broad Spectrum AlgaecideiFungicide, as part of a comprehensive cleanlinessItrearrnent program.
When applying GreenClean Granular Algaecide to soil, gravel or other similar media, incorporate the application of GreenClean Granular Algaecide into the first inch of substrate for optimum effectiveness.
To maintain clean surfaces, apply GreenClean Granular Algaecide at maintenance rates every 5-7 days or as needed to control new or established conditions.
After application, do not allow undiluted granules to remain in an area where humans or animals are exposed.
Non-target plants will suffer contact burn if undiluted granules are accidentally spilled on them. Do not apply in such a way that the concentrated product comes in contact with grass, ornamentals and other foliage.
Do not tank mix with aquatic herbicides or algaecides containing copper or bromides.