Green Water Rambling

By this point we all know “green water” is a good thing, or at least we know it is not necessarily a bad thing. Should you use green water? Like all things there is no easy yes/no answer, the answer is always “it depends”. I have not explored the color enhancing effects of green water but it is purported as such. Well, I have looked into it a little but not enough to offer up any results. How much the fish actually ingest and how much color enhancement we actually get based on different varieties of algae would be a fun study. I guess we can start there. Green water is a micro algae, there are many different types of micro algae. A few that I have played around with are Nannochloropsis, Chlorella, Synura, and whatever grows wild at my home. How do you play with algae? Well, just like anything else, feed it, grow it, feed it to things, etc.

Why Does the algae variety matter?

From Wikipedia
“It has been estimated that about 200,000-800,000 species in many different genera exist of which about 50,000 species are described.[4] Over 15,000 novel compounds originating from algal biomass have been chemically determined.[5] Most of these microalgae species produce unique products like carotenoids, antioxidants, fatty acids, enzymes, polymers, peptides, toxins and sterols.

That last bit tells us that different algae are going to have a different effect on my fish, and/or the daphnia I feed it to and in turn my fish. Nannochloropsis specifically sounds very promising as a color enhancer.

“The algae of the genus Nannochloropsis differ from other related microalgae in that they have chlorophyll a and completely lack chlorophyll b and chlorophyll c. In addition they are able to build up a high concentrations of a range of pigments such as astaxanthin, zeaxanthin and canthaxanthin.[5]”.

Nannochloropsis comes in fresh and saltwater varieties, also I seem to recall the environment they are grown in and stress you put on them can affect the various compounds they develop.

What about water quality?

Just like aquatic plants in your aquarium, if you provide them the building blocks for growth, they are going to destroy ammonia, nitrite, and nitrate. In your average planted tank you want to feed your plants macro nutrients KNO3 (N), KH2PO4 (P), K2SO4 (K), and trace minerals ala Seachem Flourish. In a goldfish tank you probably do not need to dose KNO3 as nitrate is a bountiful byproduct of our bubbly buddies. Add lights, co2, and soon pruning plants will replace scrubbing algae as your weekend chore.  But if we want the algae and not the higher plants we can tweak our inputs. The basic formula is Goldfish + Water + High Light = GreenWater.  More light or fish will fuel faster growth. Notice we are not dosing any macros or micros, the required nutrients come from fish waste. Just like pruning aquatic plants encourages new growth, you want to keep it growing fast, using a water change to dilute the green water will cause it to reproduce rapidly as fish waste accumulates.

So why change water at all?

Well for one, we want to see our fish. On one occasion I let my green water pond go too long between water changes and found the reason I was not seeing my fish was because they were sitting on the bottom full of flukes.  Another less severe case ended up with some fin rot that could have been prevented with a cleaner pond.  As your algae density is building up so are bacteria (good and bad) and other pond nasties. The algae also produce an insane amount of oxygen during the day causing your sponges filters to float and your pH strips to give scary off the chart readings. Then at night the algae consumes oxygen so without strong aeration your fish will suffocate, I have not experienced this yet. The pH changes from varying the amount of oxygen is not detrimental to fish.

After all my experimenting, this is how I like to use green water.  Fill up the pond/tank/tub with fresh water, add filters, add airstones if needed. Scoop about a gallon or so of water from an existing green pond and pour it in.  Add your fish. Depending on much sun or artificial light your ponds receive they may remain clearish or become pea soup within a week or two. Now you can adjust your water change schedule to suit your needs, 50% WC with a thick algae bloom will not help water clarity at all, on ponds like this I shoot for more like a 90% WC because the algae will come back with a vengeance.  On shadier ponds I like 50% water changes every couple weeks because I can still see my fish. Also, by “pond” I mean stock tank or small kiddy pool, large pond management is a different story. 

If you just want to grow green water, a bucket of old fish water out in the sun and old lettuce should get your culture off to a good start.  There are real scientists who work on this stuff all the time and have developed different growing mediums for different algae species. Bold’s Basal Medium is a good rabbit hole to go down, you can also buy culturing medium and different species of algae from I tried making my own nutrient mix for algae but it got too complicated and it was just easier to buy it. I buy Micro Algae Grow from Florida Aqua Farms. 

Yup, even manure is good for starting a culture

Another fun trick is to inoculate a green water pond with daphnia and forget about them, soon the water will be clear and full of fish food. You can also get daphnia cultures from or just search Ebay. That’s it, try creating fish food from bottom of the food chain up, and use algae as a tool instead of fighting it.


Further Research:

  • Which types of algae are best for food?
  • Which types of algae are best for water quality?
  • Which types of algae are easiest to culture?
  • Which culturing methods work best?
  • How to ID Algae (microscopy)?