Algae is caused by having an imbalance between light, nutrients and CO2. I think what you have going on is a bit of a perfect storm to be honest on those elements.
10 hours of light is too much, if you don't have plants now drop it down to 6 or less and dim the light if you can. The fish don't really need it, its more for your pleasure.
You are also not changing anywhere near enough water, 15% a week on a small tank with gold fish in it just simply not enough. The water will have loads of waste organics (nutrients) in it from the fish and the algae will be feeding off it.
I think you have too much fish for that volume of water. The general rule for fancy goldfish is at least 60L per fish and a minimum tank size of 100L, so 3 fish = 180L tank. The fish should get big, like the size of your hand plus fingers and live for the best part of a decade if they are being looked after correctly. They also need a lot of food and poop a lot, hence the tank size needed.
You can do a blackout to try and get rid of the algae, turn off the lights and put some thick blankets over it and no peeking for at least 3 days. Not even to feed the fish.