Hydrogen Peroxide is used by some ponders to do several different things from reversing potassium permanganate treatments to de-chlorinating water to increasing oxygen in an emergency. It's also used by some to control string algae. This page covers my experiences using HP (hydrogen peroxide) to control string algae.
How do I get HP?
It's the same stuff you find in the drug store. 3% solution, about $.75 a pint. Normally comes in a brown plastic bottle.
Is HP safe?
There's no data on long term effect on fish no matter how much HP is used. Could be safe, could cause problems. Most experiences discussed on the web are about single event type doses. Controlling string algae requires weekly doses or at least repeated doses because the string algae will keep growing back.
HP in high enough concentrations can damage tissue. Pouring HP directly onto to the gills of a fish would probably do a lot of damage, maybe kill the fish. When placed in the pond it breaks down into water and oxygen. How fast the break down happens is difficult to say. It does need something it can react with like catalase, an enzyme in some bacteria, plants and blood cells.
Whether you should use it only you can answer. Some ponders would never add anything but water to a pond. Some ponders have such a bad string algae problem they're considering filling in their pond.
Is string algae safe?
In reasonable amounts I believe string algae is great. It can clear green water, I believe, and provide food and shelter to fish. But it can grow to the point of completely filling a pond so that fish have no place to swim. When large amounts of string algae die it can really cause problems. It can float to the surface reducing the transfer of oxygen to the water at the same time using a great deal of oxygen as it decomposes. String algae that is on the surface really needs to be pulled out of the pond as soon as possible.
Normally pulling string algae out of a pond is the best method of control. But it needs to be done weekly in most cases. In small ponds that have lots of plants string algae can really be a problem to remove manually.
Does HP kill string algae?
There are hundreds of different kinds of string algae. I've had at least 2 kinds in my pond and HP killed both kinds. Some people on the web have said it didn't work for them although few details are ever given.
Dumping some HP directly on some string algae should tell the story. It should turn gray or white within hours or at least within a day. Some times it seems to disappear completely but that's not normally the case.
It does not work on green water. I have no idea why not. Maybe it does on some kinds of unicell algae or maybe at higher doses. But from my experience it had no effect.
Some people report results using as little as 1pt/1,000gal (1 pint per 1,000 gallons of pond water). Some people see no results using 1pt/500gal. The highest amount I ever used was 1pt/50gal and there was damage to Pennywort (plant) stems but the plant lived. There were Mosquito Fish in the pond and they seemed unaffected, at least none died that day or in the following year. I backed off to 1pt/100gal as my max dose.
The most commonly quoted amount on the web is 1pt/1,000gal. Unfortunately this is repeated 95% of the time by people who have never used HP to control string algae. I believe this half truth started by people who read Doc Johnson on KoiVet.com saying 1pt/1,000gal was safe. But he was talking about using HP to raise oxygen in an emergency, not weekly doses.
I used HP in a single small pond, about one hundred gallons, for almost 2 years. The pond had a waterfall, stream, plants and Mosquito Fish. I started out with a lot of string algae in the pond and added 1 pint of HP. I saw some reduction in string algae in hours but a lot remained. After 5 days I added another pint. 5 days later another. At this point there was little living string algae. From then on I varied the dose anywhere from 1/4 pint to a pint once a week to keep the string algae under control.
The last place the algae would die and the first place it would grow back was on the waterfalls. I don't know why it's stronger there but this seems true for most ponds. So I used the amount of algae on the waterfalls as a gauge for how much HP to add. I believe string algae keeps water clear by killing unicell algae (causes green water) so I don't like to kill 100% of it, just control.
If you decide to try HP I suggest you continue weekly treatments even if you don't see string algae. If you wait for it to appear or get out of control and then kill it you will get a build up of dead algae which can be a problem to remove. Best to remove as much as you can before the first treatment so the HP used will be more effective and less dead algae has to be removed. It's much easier to remove string algae before it dies than after.
Because of the amount of HP needed each week it's probably an unreasonable treatment for ponds larger than a few hundred gallons.