Water Plants and algae
Ah, spring! Color returns to our yards and fields in a glorious burst. Nature quickens its pace, and we start to make lists and plans for outdoor projects and activities. It's a pleasure to simply walk outside and breathe in the soft, fragrant spring air. The heavy rains of February have eased our guilty feelings for enjoying a bright, warm, sunny day and we feel happy with spring's promise of beautiful days. Perhaps dogs express the feeling best - with a vigorous, no-holds-barred, open-jawed, good-old-fashioned romp and roll in the wet green grass.
Well, there's a lot of excitement and activity occurring in our ponds also. Koi are reacquainting themselves with the full reaches of the pond and arebeginning to eat more. Males will soon be prodding the gravid females to release their eggs. (Assuming they're all at least 12 to 14 inches in length- they are not sexually mature if they are shorter than that.) Water plants are beginning to sprout and frogs are chorusing amorously. Of course, all this action does not come without a price - a spring algae bloom.
About spring algae blooms
Most ponds that experience a spring algae bloom do so because the biological filter is not conditioned for control of the rapid increase in organic waste levels in the warming pond. Active fish are eating more and putting more waste, i.e., ammonia, into the pond. During the winter, the nitrosomonas and nitrobacter bacteria in the filter die off for lack of nutriment. Somebacteria continue to live on the fish, however, and may recolonize the filter as waste levels increase, in a process that takes seven to ten weeks. The problem is that algae will colonize a pond with rising organic waste levels in ten days. Once algae are established in the pond, their presence makes it difficult for the bacteria to establish themselves in the filter, because dead algae cells are continually coating the filter medium upon which thebacteria live.
This discussion is focused on the average koi pond in which there is a higher concentration of fish than would occur in nature and where the fish are being fed on a regular basis. In nature, water plants and natural levels of nitrosomonas and nitrobacter bacteria are able to remove the relatively low level of ammonia and nitrite, and anaerobic bacteria in the bottom silt break down the remaining nitrate.In the average koi pond there are not enough bacteria and water plants to do the job.
Overfeeding is a major source of excess fish waste in the water, not to mention the uneaten food pellets deteriorating on the bottom. Without a properly functioning biological filter to house additional bacteria, ammonia levels build up, and algae, whose spores are carried in on the wind, colonize the pond. They take up the ammonia,thus detoxifying the water and making it safer for fish, but in the process the algae turn the water green and destroy pond aesthetics.
To put it another way, algae are a safety net for removing toxic organic waste material from a pond. If nitrifying bacteria are not present in sufficient number to break down ammonia and nitrite, algae will do the job free of charge. All they ask in return is that you not see your fish.
Algae and your filter
What all this means is that the best way to control a spring algae bloom is to ensure that the bacteria in your filter are thriving and ready for the rapid increase in waste material as the pond warms up. Clean your filter thoroughly and regularly and continue to add vitrifying bacteria, such as Aqua 5. You may also add, as a preventive,an algicide such as Simizine to inhibit the growth of algae. It is much more efficient and effective to use an algi cide as a preventive at, say, half dosage, rather than as a cure for a full-blown bloom. Also, please note that it is not a good idea to simply add algicide to a fish pond without a biological filter. Without a filter or algae to take up ammonia, your fish can be poisoned by it, as it is toxic in as little as 0.15 parts per million in its free form.
Water plants and algae
Water plants help control algae to some extent because they also take up ammonia and nitrite as nutriment. However, water plants are always dropping their leaves and other organic matter in the pond, thus improving conditions for algae. One of the cleanest water plants for algae control is water hyacinth. Marine salt added to the pond in about a 0.1% solution also helps control algae. Water changes are only a temporary solution, as algae will recolonize a pond containing new water in about ten days.
Algae change the chemistry of your pond water by taking up oxygen at night and releasing oxygen during the day. These alternating low and high levels of oxygen can stress and even kill fish. Algae also cause pH to rise, thus in turn causing a higher ratio of free ammonia to ionized ammonia which is not good. High pH also makes it difficult to safely transfer fish in or out of your pond, as fish are severely stressed by rapid pH changes.
Finally, there are literally thousands of types of algae. The single-cell types, which make the water look like pea soup, are generally easier to control than the long, stringy filamentous varieties, which grow on the sides of your pond. Algae provide good nutriment for fish, and a special type, spirulina, are added to high-quality koi foods for color enhancement. (It's also sold in health food stores for koi keepers' personal use.)
In summary, algae growth is a complex problem for pond owners. The best control is a clean filter with a thriving bacteria colony. It's important not to overfeed your fish and don't allow uneaten food to sink to the bottom. Algae provide some good for the pond, but a few algae go a long way.