Hi Madeleine I would recommend bringing in a sample of tank water along with tap water. As this issue may be a result of a few different things happening. I wouldn't recommend algaecides as that is a temporary fix and can be harmful to fish and inverts.
It's the best place for all your aquarium needs!!!!
I'm with Mark, direct sunlight or too much artificial like is usually the cause. Algae is a plant that thieves in sun light. I don't believe in chemicals as much as finding the main cause and dealing with it directly. Also, do you leave lights on around the clock?
If the sun is hitting it consider moving it or shading it. Con sider reducing artificial light. If you have fish in the tank you might be overfeeding.
Get ALGAEFIX. Put 6drops in a 20 gallon size aquarium..Do it every 2 days for a week. Clean filter. Do then 1 time a week for 3 weeks. Clean filter. Then occasionally if needed
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