Hobbyist goldfish breeders and enthusiasts

Growing Goldfish in Green Water

Summer is a great time for goldfish. Our spawns from early spring have been outside enjoying the sunlight in 110 gallon stock tanks getting fat and happy. The warm weather, lots of space, and long days of sunlight does wonders for their growth and development. There’s an added benefit to being out in the sun though and that’s “green water”. Growing goldfish in green water provides a free and easy boost to the development of your goldfish.

Inside, in a display tank positioned near a window green water is the bane of many aquarium hobbyists. This is understandable, as the water can make it hard to see and appreciate your fish and to the uninitiated visitor it may appear you’re not properly taking care of your fishy friends. Beyond what many may see as undesirable aesthetics though green water is not only harmless to your fish but can actually be quite beneficial for them.

What the heck is green water?

“Green water” refers to having some quantity of a variety of green microscopic algae resulting in the water taking on a green tinge. Green water is particularly prone to occur in any water kept outside such as your koi/goldfish pond. However, being that the biggest influencer on having green water is sunlight, you may find it happening in your aquariums that are placed near windows that receive direct sun for a good portion of the day. When we talk about green water we’re specifically talking about algae that remains suspended in the water column, not things like hair algae growing on the side of your tanks or ponds.

Growing Goldfish In Green Water

How do you get green water?

At this point you’re probably asking how you get green water, or maybe why you have green water. As I’ve already mentioned really the biggest factor is sunlight. The algae in green water is a plant and thus it performs photosynthesis, in order to do that it obviously needs lots of light. Eight to twelve hours a day such as natural daylight hours seem to produce the best, but anecdotally people trying to cultivate green water indoors under artificial high intensity light have found even five hours sufficient.

A lot of people spend a great deal of time, energy, and money on things like UV filters to try to combat green water. In a lot of cases this is because the green water can make it a lot more difficult to view and appreciate your goldfish. Sometimes people may even mistakenly believe that green water is dirty or bad for your fish. However, not only is the green water not bad for your fish but it can be quite beneficial.

How does green water help with growing goldfish?

The microscopic algae in green water is the bottom of the food chain in most natural ecosystems. Zooplankton and bugs feed on the green water, fish feed on those, and then bigger fish and mammals feed on the smaller fish. Outdoors having green water can help cultivate other larger food sources for your goldfish especially in a pond. Goldfish appear to also be able to eat the algae directly.

This algae is extremely high in protein and provides great natural color enhancing pigments. The protein helps your fish grow as it provides a small constant (free!) food source for them to consume at their will. Additionally that protein helps promote excellent wen growth in breeds such as orandas and ranchu.

You really have to see the difference green water makes when it comes to color to believe it. Check out this thread on the Goldfish Keepers community to see a lot of great before and after photos and video to see for yourself. A lot of goldfish foods especially the color enhancing varieties contain Spirulina powder. Spirulina is one of the most common algaes that contribute to green water.

The algae in your green water feed off of the CO2 and nitrogen produced by your goldfish as waste. In fact it could be argued that excessive and fast growth of green water is a sign that your existing biological filtration isn’t sufficient. Having algae in your water means they’re absorbing CO2 and ammonia and producing oxygen through photosynthesis. This is free extra filtration to keep your water parameters better and more stable.

In Conclusion


  • Free natural supplemental food source
  • Increased overall growth
  • Increased wen growth
  • Enhances goldfish color
  • Improved water quality
  • Reduces incidence of swim bladder disease (SBD)


  • Aesthetics
  • At night uses more oxygen so proper aeration is required

Should you have green water?

Our primary focus is on breeding goldfish. Considering how many tanks and fish we have, we don’t do anything to try to combat green water. Even for our tanks indoors we don’t do anything to fight (good) algae or green water, it’d be a (costly) fight we couldn’t win and we like all of the benefits it has for the fish we’re keeping and grooming. Growing goldfish in green water helps produce bigger fish with much more vibrant color.

That being said, at the end of the day if you embrace or fight green water (and other algae) really is just a personal preference and one that primarily comes down to aesthetics at that. If you’ve got one big display tank in your living room that you want to be able to show off to friends and family, well I can definitely understand why you may want to keep it algae free.

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