My tank used to look like that also and my Mom's tank looks a little like yours as well. I think you should do a 20% water change every week and also maybe adding some calcium or kalkwasser mix. Your tank is really small so you dont need much but it helped mine and you will slowly see some improvement. You don't have corals that will soak up the calcium but just a little maybe every 1 to 2 weeks might do the trick. My best bet would be the kalkwasser because you can add it when you do a water change. You onle want to use the clear water that is remains when letting it set. Do not add the powder to your aquarium. Also, another thing is that your lighting could be causing all that algae. A higher wattage for maybe only 8 hours a day would be good.
