will it hurt fish to put apple cider vinegar in to kill algea? i have a small yard pond and have Koi and Fancy gold fish.. we get a lot of green water and algea.

Hi Ann, Welcome to ATP!

I don't have a pond and don't know a thing about them but I'd be wary of adding vinegar to the pond without knowing the proper amount for the quantity of water. If you have fancy Koi and other fish you sure don't want to harm or kill them in the process of trying to control algae. Hopefully someone more knowledgeable about backyard ponds will be along with some advice for you soon. Meanwhile, I googled and found this information regarding pond algae control that might be of help:

http://www.drsfostersmith.com/...

Again,

Lin

The amount of vinegar you would have to add to affect the algae would be harmful to your koi, which require a balanced pH. Also, vinegar is a quickly metabolized nutrient in a mature pond, and will lead to increased microbial growth, which is probably not what you're looking for. Although there are chemicals that you can add to kill algae, they will destabilize the ecosystem and are only temporary measures.

Most effective is to lower the phosphate and nitrogen load in the system. This means not overfeeding, removing decaying debris, and exporting nitrogen (fish waste) somehow. Options include active filters, plant life, nitrogen absorbers such as straw bales, and water changes with dechlorinated water.

Come visit us in the Ponds and Water Gardening forum for advice. Algae is a common issue, and there are many ways that our members handle it. We'd love to hear about your experiences.
http://garden.org/forums/view/...

thank you to all that answered my question and have a blessed and happy day

Wouldn't a barley straw ball provide control without harming the fish?

Studies on barley straw have been inconclusive. Google and look at the various university studies on the subject.

What most people can agree on is:

Barley straw is not effective on existing algae, only in preventing new algae

It is only effective on certain kinds of filamentous algae. No specific chemical or mechanism has been identified for this ability

The straw must be fully dried, and not hay, or it will add nutrients to the water and worsen the algae problem

The straw will still break down in the presence of nitrogen products, adding net nutrients to the water and encouraging plant and algae growth if it is not eventually removed

Other than the long range addition of breakdown nutrients, it is not likely to be harmful to your pond, so is a safe experiment to try

THANKS EVERYONE