Been battling green water algae since I so brilliantly cultivated it a couple of months ago with a total tank replant, practically turning the substrate upside down plus leaving 4 39w T5s on for nearly 30 hours when I fell asleep for 12 hours during a little "break" in the new planting. Luckily, I had placed the light fixture back on the tank before my nap to admire my progress or else the green water algae might not have flourished so.
I tried the "one-two" punch as soon as I finished reading the OP yesterday: 16 tbsp (250ml/9oz) of H202 in my 55 allowing about 40+ gallons since I have rocks and also elevated substrate as a couple of "hills" on the sides. Ran my Eheim Ecco Pro Easy 80 empty of media baskets plus my Rio 400 full blast with the duck bill super thrust nozzle for crazy circulation. After 15 minutes I used both of my siphon vacuums to do a 50% water change with pristine RO water. I added the Excel when the tank was half empty and had it filled within 5 minutes - total water change was less than 20 minutes since I had 4 5 gallon jugs of water ready and so only had to fill one from my holding tank which is only about 30 steps away.
My 50 neon tetras, 8 Coreys, 5 green tiger barbs and 2 Thai algae eaters did not seem to dig it not one iota. Upon the shock of the H202 dump the barbs immediately quit pestering each other, the neons balled up into a still life of a school in the center of the tank, the algae eaters immediately quit begging for food and dashed into a thicket and all the coreys stopped wriggling around on the bottom and just froze where they were. I forgot to check out my assassins at the time to see how they were doing but they're here today along with all the others.
Today I have recharged my Vortex D-1 and will run it while scrubbing all the remnants of the (DEAD?) GWA from the glass and rocks. So we'll see.