Algae Control

There are two kinds of algae and they need opposite answers. Green water floats, and a UV clarifier fixes it. String algae clings to rocks and waterfalls, never passes through the UV chamber, and a bigger UV will do nothing for it.

String algae is a nutrient problem, fed by fish waste and overfeeding. The algaecides and barley products here buy you time while filtration and feeding habits do the real work. Ironically, ponds with excellent UV often get more string algae, because the free floating algae is no longer competing for the same nutrients.

Pond algaecide bottle, a burlap pouch of barley straw and a wet rock trailing green string algae beside a koi pond, the two approaches to controlling pond algae.

Worth knowing before you buy

Ground rules and answers from the ponds we run every day in San Jose.

Turn the UV off while you medicate

UV-C degrades most treatments, and activated carbon strips them out entirely. Bypass both for the duration of a course, then bring them back online.

Common questions

My water is green. Do I need a UV?＋

If it is pea-soup green and you cannot see your hand a few inches down, that is suspended single-celled algae and yes, a correctly sized UV is the reliable fix. Most ponds clear within three to five days once it is running. Plumb it after the filter so the water reaching it is already clear, because UV-C cannot penetrate dirty water.

What a UV will not do is stop the algae coming back if the underlying cause is untouched. Green water is fed by nutrients from fish waste, overfeeding and sunlight, so a UV is a control, not a cure.

I have stringy algae all over the rocks. Will a UV fix that?＋

No, and this is the most common misunderstanding we hear. String algae is filamentous and it clings to surfaces, so it never passes through the UV chamber. Running a bigger UV will do nothing for it.

String algae is a nutrient problem. Pull it out by hand where you can, cut back on feeding, keep up water changes, and use an algaecide or a barley product to buy yourself time while the real fix takes hold. Ironically, ponds with excellent UV often get more string algae, because the free-floating algae is no longer competing for the same nutrients.

I am not sure what I need. Can you just tell me?＋

Gladly, and this is genuinely the fastest route. Send us your pond volume, your depth, roughly how many koi you keep and how big they are, your plumbing size, and the height from the pond surface up to your filter return. That is everything we need to size a system properly.

We have been building and maintaining koi ponds from San Jose since 1998, and we run this gear on our own systems. We would rather talk you into the right pump than sell you the biggest one.

Have us size your systemFull pond guide & calculator