HI folks, I have a problem, an algae problem. Please take a look at my pics. I set it up as a hospital tank left it a month and my gf made me put a female Betta in there on her own even though I thought it was a bad idea because the tank probably wasn't cycled properly, I think it's only about 15 or 20 litres. We went on holiday for 1 week, before we left I did a 30 percent water change. I have the lights on a timer for about 6 hours a day it's just 8 LEDs but lights the tank very nicely. When we came back it was like the picture I'm showing you but 50 times worse it looked absolutely mental in there just after 1 week. The Betta got ick, I cleaned it did a big water change put some general fish ointment in there forgot what it's called I think it's just mainly salts. I did a few water changes over the next couple of weeks but the fish died ☹. So I emptied the tank completely cleaned everything including the gravel but kept the main sponge in the filter to retain any good bacteria. I've also added some water from my fairly well established aquarium to give it a boost. But since then it keeps growing this weird algae that's really dark green and spreads like wildfire. I know smaller tanks are harder I've kept a similar sized tank before and that went really well
Cyano bacteria, also called blue-green algae. You need to do a total black out for a few days along with manual remove and water changes to fix it. Also adding floating plants (duckweed, water spangles, red root floaters etc) will help soak up the nutrients so the algae can't use it.
If that doesn't work you can even use an antibiotic or hydrogen peroxide to kill it off. Google can tell you more for that.