Blue Dye – A Fish Farmer’s Point of View.

East Yorkshire Fish Farm Sunday Blog

Hi Pond Keepers!

I’ve been asked to write this quick blog on the controversial topic of using blue dye in ponds to stop algae and blanket weed. The reason I’ve been asked is because, for the last ten years, I’ve owned and managed East Yorkshire Fish Farm—after spending 15 years before that as a farm hand, working my way up the hard way. In all that time, I’ve come across many strange, bizarre, and wonderful things around our ponds. Over 25 years, those ponds have stretched across three sites—each over ten acres—a canal we ran back in the ’90s, and many emergency callouts for our long-time customers.

To the topic at hand: blue dye. This year—2025—has had an unusually hot spring, and the abundance of sunlight has caused more ponds than usual to turn green. It seems every day I get two or three people ringing me, asking if they should add some kind of treatment to their pond. This is a big increase from the usual one or two enquiries per spring in past years. I guess that’s why the koi world is buzzing with talk about blue dye.

So, what does it do? And how does it work? Well, in layman’s terms, it blocks sunlight from entering your water. It creates a barrier that stops any kind of photosynthesis.

Think of sunburn: a little sun doesn’t burn your skin, but too much does—so you apply sun cream to block it. Same with a pond. Too much sun, and it goes green. So you add blue dye to block the sun. The green water stops. Well, not exactly—it actually dies off. People think it just stops growing, but it doesn’t. It dies because it has no food, no sunlight, no photosynthesis.

Great—no green water or weed, right? But think: what lives on that wonderful phytoplankton (algae) we’ve just killed off?

Answer: Daphnia.

Daphnia is the main thing I want to talk about, but there are so many insects and critters that koi eat that rely on phytoplankton. Carp prefer these natural foods over any preserved pellets. But back to daphnia—the number one best food for any omnivorous fish.

Koi carp, mirror carp, common carp—they’re all omnivores, meaning they’ll eat just about anything. I can’t emphasise enough: daphnia is the top food for carp. It’s high in protein, alive and fresh, and so small it’s like it was designed perfectly for a carp’s digestive system. Or maybe the carp evolved perfectly to eat it. You see, carp have no stomach—just intestines. One reason they’re always hungry is that food passes straight through like a conveyor belt. If something heavy or large clogs it, problems arise. But daphnia, being so small, is absorbed quickly—no risk of blockages.

As a fish farmer, my main goal is to run a successful business. For that to happen, my fish need to grow—and to grow, they need room and food. Daphnia not only makes excellent food, but when it really takes off in a pond, we farmers lime and empty ponds specifically to make it thrive. The result? Clouds—dense masses of live food for carp to hoover up like baleen whales do in the ocean. (If you’re wondering what I mean, just watch Blue Planet. Every fish keeper should.)

This nearly-free food helps our fish grow faster and cheaper than anything else. Unfortunately, it doesn’t last forever—April to June gives us three golden months. Free food, grown by sunlight. Without it, my business wouldn’t survive. I can’t throw that away just for the sake of a bit of blanket weed or cloudy water for a couple of weeks.

Now, I understand ponds can get ridiculously overgrown with weed in spring. Bad enough that people panic. But I’ve always found it happens at the start of the growing season—especially in ponds that are understocked and shallow enough for sunlight to reach the bottom. I say understocked because, as I mentioned, I grow fish—carp need room. In April, I might put 400lb of carp into a one-acre pond—loads of space and clouds of daphnia. By June, the natural food’s gone and the fish have doubled in size, so we start supplementing with the best pellets.

Still, keep in mind: even the best pellets aren’t as good as naturals. We push the fish to grow up to the biomass the pond can support—not based on natural food availability, but based on oxygen levels. In the wild, natural food limits the fish. For us, it’s oxygen.

As the fish feed and root through the pond, the mud kicks up and stops light from getting in—just like blue dye would. That’s perfect, because by October it’s time to empty the pond, remove every last fish, lime the bottom, and restart the whole natural cycle.

So, if you manage your stocking levels right, weed shouldn’t become a problem—your fish will do the job for you. In fact, weeds can become an asset—providing warmth, food, protection, and natural water filtration.

To sum up: I’ve tried to explain as simply as possible why blue dye just isn’t used on fish farms. It’s not used because, quite simply, we farmers don’t.

“Take away the sunlight to help pond life.”
Think about that. The sunlight—the one thing without which there is no life on Earth. Take that away to help pond life? Just let that sink in. If you take nothing else from this blog, take that.

Yes, in some rare instances blue dye could be used, and it can work for a specific outcome. For example, in a big lake that isn’t fishing well, and the owner can’t afford to restock. If you kill off the food source, the fish may resort to feeding on the anglers’ hooks. They’ll probably end up skinny and die in winter from lack of body weight—but that’s a later problem. Sounds idiotic? I’ve seen it far too often. Not surprisingly, they end up buying more fish the next spring—from the same person who told them to starve their pond.

Another valid use (and I believe this is its original purpose) is in ornamental setups—to stop blanket weed blocking pumps or prevent algae stains on large fountains in town squares or royal gardens.

I suppose if your pond is overrun with algae-eaters like grass carp, roach, rudd, or sticklebacks you don’t want, and it’s too big or old to drain, blue dye could be used to take away their food source and starve them out.

Anyway, I hope this gives you fish keepers something to think and talk about when it comes to caring for your ponds. One last tidbit: for anyone with a 5,000-gallon pond or smaller suffering from weed or algae, just put a cover over it—anything that blocks the sunlight. Just like blue dye.

– Chris Heslop – Visit our website to see our carp stock. We are the East Yorkshire Fish Farm Carp Stock Specialists in East Riding of Yorkshire