Green water is caused by free-living algae that thrive on high nutrients and light. No matter how fine your filter media is, the microscopic nature of the individual algae cells means that they easily travel through the filter without being removed.
Two main aspects of this unsightly issue need to be understood in order to control it. Green algae photosynthesise and are more likely to be a problem where sunlight reaches the aquarium or situations where aquarium lights are on for too long during the day. The second issue is nutrients, with high levels of nitrate and phosphate being particularly good fuel for algae blooms.
Once you know that high light levels and a lack of water changes are generally the cause, you can get an idea of how best to control things. Green water is rare in aquaria with lots of plants and low fish populations and this shows that measures made to encourage plant growth will be useful in controlling nutrients. Often the worst affected tanks are those housing large, messy goldfish which place a lot of pressure on filtration and maintenance regimes.
Solutions to this problem can be either low or high tech. By far the easiest solution is to add an Ultraviolet clarifier (UVC) to the filtration loop. Just like the pond units, these devices kill the algae cells and clump them together without any chemicals, producing particles large enough to be removed by the filtration. In the short term, chemicals can be added which clump algae together in a similar way, but this tends to be a short-term fix. Blacking out the tank entirely by turning off the lights and covering it with a heavy towel or blanket for a few days can provide a solution but this is also a short-term solution. Reducing the day length by putting your lights on a timer can make a big difference and for aquaria without live plants, consider having your lights adjusted so that they’re on when you’re there to appreciate them, rather than putting them on first thing in the morning until last thing at night. A good old-fashioned campaign of large partial water changes can help but if your source water is high in phosphates then this may not be effective.
As with any algae issue, solving the problem means removing the cause or making a long-term adjustment. By far the simplest and most effective measure, in the long run, is to install a UVC which will only require minimal maintenance for maximum results. Just make sure that the sparkling water clarity it gives doesn’t become an excuse to neglect regular water changes.