Green water is a nasty problem in many aquariums; one day you have a gorgeous, crystal-clear tank and the next day you wake up to an aquarium of opaque green. This condition is caused by a free-floating algae bloom, and it really can go from no problem at all to being able to see absolutely nothing inside the aquarium overnight. So what do you do about it?
As with most things, the best cure is prevention. Green water occurs when an aquarium is overstocked or has too much decaying organic matter in it. This process creates an excess of nitrates and other nutrients that feed the algae. Once all the right environmental factors are in place, the algae takes over quickly and mercilessly. The key is to keep nitrates down and the water parameters healthy for the fish.
You can start by ensuring you're not over-feeding your fish. A fish in the wild can never be certain where its next meal is coming from, so its metabolism has adapted into that of an opportunistic eater. Healthy fish will always be hungry, so you can not simply feed them until they stop eating. Give your fish just enough that it takes them about two minutes to eat all the food twice a day (three times if you have juveniles); if you see uneaten food get to the bottom, cut back a little.
Next, make sure you have good mechanical filtration and a well-aerated tank. For additional preventative, you can use a time-released water maintenance product such as EcoBio-Block products. These products last up to two years apiece and ensure that the water in the aquarium stays perfectly balanced by breaking down the toxic ammonia and nitrites from waste and excess food, as well as re-supplying essential minerals in the water as they're used up by the fish.
Many aquarists believe that direct sunlight is the main cause of green water and this is not true. While direct sunlight should be avoided because of its effects on the water temperature, algae can not bloom without quite a bit of excess nutrients in the water. It is true that algae can not grow without light as well, but the algae requires very little light for its photosynthesis; once it's started, any tiny bit of natural light that reaches the aquarium will be sufficient to continue its growth.
Live plants are great for helping get rid of excess nitrates and there are many hardy varieties that will use up a lot of the spare nitrates that algae blooms feed on. In most cases you'll want to keep an eye on the plants so dead pieces can be removed before they start to decay, however this is not a problem if you have an EcoBio-stone or adult P. bridgesii (a type of apple snail commonly sold in pet stores), or "brigs". Brigs grow to about the size of golf balls, lay their eggs above the water line so population control is easy, and as adults never eat healthy plants. Be certain you have an adult before you introduce it into a planted tank, though, because the juveniles are still able (and very willing!) to eat healthy plants. If you decide on snails, do a little research first. P. canaliculata, or "canas", can look very similar to brigs if you aren't familiar with them, and canas grow to the size of a softball and will always have a voracious appetite for live plants.
Regular partial water changes are required to keep the nitrates down and remove organic material from the aquarium. Again, if you have a time-released product such as EcoBio-Stone this is not necessary, though occasionally stirring your substrate to get rid of potentially harmful gas pockets and to send organic materials through your mechanical filter is recommended. If the water is kept in pristine condition, you will not have a problem with green water.
Finally, once you have green water it is difficult and time-consuming to get rid of. You can begin by restricting light, even covering the outside of the aquarium with paper to keep additional light out, and do a partial water change to get rid of some of the excess nutrients. Note that the water changes in themselves will do nothing to get rid of the algae, it reproduces quickly and can only be gotten rid of through "starvation". Cut back your fish's food and leave the tank light off for the duration of the clearing time. It may take a couple of weeks to clear up.
As soon as the tank is clear, immediately take some of the preventative measures outlined above to avoid re-occurrence. I especially recommend ensuring that you have a good mechanical filter, plenty of aeration to keep beneficial bacteria healthy, and a time-released maintenance product to ensure the water is always as close to perfect as possible.
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