Hey Peter, A belated Welcome to the Forum!
I am assuming that this is soft green algae, and perhaps some brown diatoms, and it grows primarily on the glass?
A coarse nylon scrub pad will do the trick, diatoms may take a little elbow grease. Watch you do not get gravel in between the pad and glass, that will scratch.
But...
Cleaning it off is not going to help, it will come right back
as quickly as a few hours.
This type of algae comes and goes in an aquarium and has a definite life cycle. If it is present all the time, weeks on end, then you need to look into several things. I see it in my tank once in a while but it never stays more than a few days. And yes my tank even takes direct sunlight for a little while every day.
Unless you have plants, light should be limited. If the room is well lit with ambient light then use the tank lights only when you want to view the fish, or not more than 8 hours a day.
Regular substantial water changes and substrate vacuuming are essential. Here's the trick. These algae will release spores almost as soon as you hassle them. They will also just swim around and wait for you to go away so they can get on with it
So a substantial water change is important right after you clean the glass.
Revisit your feeding routine are you feeding too much and raising the nutrient level of the water?
You can always fall back on a 72 hour complete blackout of the tank. That will kill it all and start you at 0, you must however keep up the water changes, vacuuming, and food monitoring or it will come back in a few weeks.
Hope this helps
Regards.
Jay
but you gotta love em
