algae control

Algae spores are ubiquitous and unavoidable in a fish pond. Whether a green water algae bloom or a healthy mat of filamentous algae on your pond liner - it is a maturation process. Green water is of no immediate threat to fish but there are some concerns. Oxygen depletion at night, excess pH swing due to photosynthesis and not being able to see your fish the the main ones

What is Algae ?

Algae, of which there are many species, are plants [the exception being blue green algae which is actually cynobacteria]. Of main concern to pond keepers are both planktonic [free floating] and string algae, both from the phylum "Chlorophyta"

Single celled planktonic algae is suspended in the water and in large enough numbers, cause the "green water" appearance pictured below. String algae affixes itself to any wet surface surface and as the name implies, is a long stringy plant. Variants can also be seen floating in clumps on the surface of the water and are also known as "blanket weed"

The reality is you can't stop the growth of all algae without harsh chemicals - chemicals which preclude the option of keeping fish
This type of coverage will control alge - but you still won't see your fish
  • Green water is a pretty common occurence each Spring
  • String algae gathers heavily on nutrient rich waterfalls
  • Advanced algaes take the appearance of regular plants



How "Not" to Control Algae

Doing a massive water change is not the solution to rectifying a green water problem. All it does is reset the cycle and kill beneficial bacteria that are trying to get established. The water will turn green again, likely within a week or two. Our city water contains 12.5 ppm nitrate right out of the tap !

Another mistake people make is scrubbing all the string algae off of the liner. Again, string algae is a competitor to the planktonic algae so removing it will leave more nutrients to create green water algae blooms. It is impossible to stop a nice carpet of string algae from growing on a liner in fish pond so leave it alone, it is beneficial. A microscopic world of bacteria and filter feeders will grow and help clean your water and fish also graze on it

Algaecides, in particular those based on copper sulfate, are toxic to fish and you dose enough to kill the algae but not the fish - or at least that is your hope. As a general comment, bottled remedies are a band-aid approach and can often be detrimental

Many tout the "natural eco system" will do it all as plants and fish balance to create clear water. Of course this is possible, however, most hobbyists have ponds that contain a tremendous bio load packaged in a small closed system. Sure 70% coverage with plants will likely provide clear water - after a couple months when they finally grow in. Plants both rob the algae of sunlight while competing for nitrate and ammonium but can't be relied upon as a year round solution

The poor timing of green water usually occurs when fish observation is critical, koi are coming out of their Winter weakened state and need to be closely watched
In Summary
Leave the pond alone ! Cleaning and scrubbing the liner and doing massive water changes will just reset the cycle and never allow the pond to get established. You will kill zooplankton like rotifers, many of which are filter feeders. Plankton actually eat algae and can filter up to 1000 times their own volume in 1 hour. In the final analysis, plants are not a realistic full season approach to controlling algae - if you want year round clear water you need to invest in a UV Clarifier

Methods to Control Algae

There are many ways to limit algae growth but they essentially break down into 2 groups: The expensive but most effective approach or the less expensive band-aid approaches. That being said, a multi-tiered approach is more effective than relying on just one method

Plants, established bio filters, reticulated foam, barley straw, blanket weed controllers & powders, shade clothes, dyes, flocculants, phosphorus removers, aeration and water movement etc, all work in concert to help keep algae in check. But these should be considered ancillary approaches to the foundation of your planktonic algae control, the Ultraviolet Sterilizer

UV's are expensive but investing in one is really the only way to proceed if you are serious and do not want green water at any time. Not only will a UV kill and cause clumping of planktonic algae, it will also sterilize the water by killing bacteria and viruses. Obviously very beneficial [see the chart below]

uv sterilizer setup
As for string algae, it normally will stay in check on its own [with the help of the fish consuming it], remaining 1-2 inches in length. Manual removal and then treating with a product like "Pond Balance" is my recommendation. Waterfalls are particularly prone to string algae due to the water flow and nutrient load [nitrates] coming out of the filter. These can be covered [black tarp] and salted for a couple of days to kill the algae and provide some residual control