Blue-green algae can quickly icky up your aquarium once established. If they're growing on artificial plants, take a twofold approach to remedying the situation: manually remove algae to get ahead of the problem, but also correct the conditions that fuel growth, otherwise the stuff just keeps coming back.
Blue-green algae aren't technically algae at all—that gross stuff growing on your artificial plants is actually a colony of bacteria known as cyanobacteria. That means you'd need an antibiotic like erythromycin, not an algicide, if you wanted to medicate your problem away. However, that approach can harm other life in your tank and negatively affect water quality, so it's best reserved for emergencies. Before taking such drastic measures, focus on eliminating the qualities that promote blue-green algae growth: too little circulation and oxygenation, too much light and nitrate and phosphate-rich water.
Start by removing the blue-green algae already in your tank. Take the artificial plants out, even the ones that look clean, along with your other decorations. Scrub them down with a toothbrush under hot running water, getting into all the nooks and crannies. (Don't use soap on anything that goes in your aquarium; even a trace can harm fish and plants.) If you see algae on your tank glass, remove with an algae scraper. You might be tempted to let algae eaters do the labor for you, but few eat blue-green algae and those that do are unlikely to be thorough enough to handle the job on their own. If you do want some critters to help control blue-green algae, Ameca splendens, otherwise known as the butterfly goodeid or butterfly splitfin, and some nerite snails are worth a shot.
Still water that's low on oxygen promotes blue-green algae growth. A device that disturbs the surface of your tank's water, like a trickle filter or a decorative fountain, helps better aerate your underwater world. Other devices are available for this purpose too, like airstones and bubble wands, and many are made to look cool. Also, if your gravel or other substrate's getting clogged with gunk, it's not getting enough circulation between the cracks, which promotes algae blooms toward the bottom of the aquarium. Use a substrate vacuum at least every few days. Also, you might be going too heavy on the fish food, leaving leftovers sitting in the substrate; feed your swimmers only as much as they eat without taking food into their mouths and ejecting it, once daily.
Blue-green algae love light, especially natural sunlight. Your aquarium shouldn't be getting any direct sunlight. If it is, it's time to relocate the tank, put up some blinds or drape a towel over the side that's getting hit by the light. Unless you have plants or coral or other life with special needs, there's no reason to leave your aquarium lights on for more than about seven hours per day. Also, if you've been using the same light bulbs for more than six months, replace them. The blue-green algae on your artificial plants undoubtedly prefer the light wavelengths right where they are. A bulb's wavelengths alter over time, so a new bulb's will be different.
Like all life, blue-green algae require nutrients to grow, reproduce and survive. They are particularly fond of nitrate and phosphate, but other compounds that can build up in the water, like ammonia, will do just fine. Increasing your water circulation is a good start, as is preventing uneaten food accumulation and cleaning your substrate regularly. Also, make sure your filter and protein skimmer are working well and that you're cleaning and maintaining them as the manuals tell you to. Remove dead fish or plants from your tank right away; they quickly start to decay, a process that gives off the nutrients you need to keep in check. Don't forget the all-important partial water changes; replace 10 to 15 percent of your tank water every week or two to help maintain appropriate nutrient levels.
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