Blanket Weed

The reason that Blanket Weed is such a usual problem in the summer for Koi keepers is due to the fact that it grows very quickly in sunlight and is fed by the nutrients from fish excrament. There are a few other varieties of Koi pond weeds which are also filamentous algae and the most usual of these are Cladophera, Rhizoclonium, Vaucheria and Ulothrix (Ulothrix is a Chlorphyte like Spirogyra and looks very similar). Blanket Weed is a long thin filamentous alga, and like Ulothrix thrives in Koi ponds.

In Koi ponds which tend be well stocked with fish, the tendency to overfeed creates additional fish excrement which contains nutrients favoured by Blanket Weed - particularly nitrates and dissolved ammonia. Blanket weed loves a high pH and phosphates in the pond water. Ponds may be free of Blanket Weed for years and then suddenly become infested. There are two main algae growths, green water and stringy 'blanket weed', usually if you have green water then blanket weed doesn't grow well as there is less sunlight but if you have clear water then the blanket weed will grow and keep the green water away.

Blanket weed is really difficult to remove from a Koi pond as it clings to rocks, plants and anything else else. It isn't harmful in itself, in fact it will in fact improve the water quality for the fish in the same way as other plants will. It spreads and recolonises vegetatively, by branching out and breaking off parts of its own structure which then colonise new areas.

Blanketweed growth rates may be staggering, and this problematic pond infestation can be controlled in several ways. When a weed killer is used, it will routinely kill the blanket weed but the chemicals soon dissipate. The big draw back with any of the chemical additives either man made or natural is that they do not work in every Koi pond and may not even work from one year to the next as the algae gradually becomes immune to the chemicals.

Once the pond has been cleared it's a good idea to vacuum the dead algae from the bottom of the pond as this will stop the nutrients from re-entering the cycle. Introduce more natural plants like lilies to your Koi pond as this will stop light penetrating the water and will compete with the algae for nutrients.

One of the greatest ways to control algae problems naturally is to minimize the amount of phosphate and nitrate in the water. You should aim at achieving a natural ecological balance in your Koi pond, add more plants, don't overstock your fish, and top quality water is essential. Another natural control of algae has been the production of plant extracts the most common being barley straw. It can take a little effort and time to remove blanket weed from your Koi pond but it is well worth it.