I tried this method yesterday, and while all my (black brush?) algae now looks weakened and pathetic, all 7 of my juvie BN plecos died within 12 hours of the treatment.
50 gallon low-tech planted tank, estimated 40 gallons water
Added 16 tbs H2O2 (4 tbs per 10 gallons - I figured none of my livestock were all that delicate, so after I added 2 tbs per gallon and no one looked stressed, I bumped it up to the "full" dose. Stupid, stupid me.)
Ammonia, nitrites = 0; nitrates below 50 ppm
TDS ~ 900
pH ~ 8.0 (Tap water here is close to 8.3, 8.4, and I've been slooooooowly adding SeaChem Neutral Buffer because I wanted to get my pH close to 7 to increase the odds of BN breeding.... too bad I murdered all my breeder BNs)
Java fern, on driftwood
Small Amazon Swords
Egeria/Elodea densa (anacharis)
5 brown BN Plecos
2 albino BN Plecos
1 old, scraggly momma guppy
?? week old guppy fry
1 male betta
1 one-eyed Black Moor goldfish
Rena XP2 canister filter, turned off for the 15 minute treatment
Marineland Penguin Powerhead (170 GPH) running two sponge filters
Here's what I did:
Step 0) Prepare approximately 25 gallons of dechlorinated replacement water, in buckets next to the tank, waiting for showtime.
1) Turn off canister filter
2) Remove sponges from Penguin Powerhead so it's just providing water flow, no biological filtration
3) Turn off air to bubble strip
4) Add Hydor Koralia Circulation Pump (rated 850 GPH - you said good flow, I got good flow!)
5) Add 10 tbs peroxide (approx. 3 TBS per 10 gallon estimated volume)
6) Wait 5 minutes, constantly re-orienting powerheads for full "washing machine" effect
7) Add 6 more tbs peroxide, since nobody looked stressed. This is the step I most regret.
8) After 17 minutes (I was aiming for 15, but don't think this is all that far off from the goal), I turned on the air pump to the 18" air strip, and added the sponges back onto my Penguin Powerhead, to allow some of my good bacteria to start processing the peroxide. I also removed the Hydor Circ Pump, since it was literally flattening most of my wisteria.
9) Drain 1/2 volume of tank into almost literally any random watertight container I could find, since I was aiming for a speedy water change, and all of my water-change buckets were full of replacement water.
10) Start adding replacement water. I kept an eye on the thermometer, and even with the influx of room-temp water, the temp never dropped more than two degrees. The heater was plugged in and on for most of the water change process.
11) Turn on canister filter, so full arsenal of good bacteria can start processing the H2O2.
12) Add slightly over 1/2 of Flourish Excel dose (i.e., 3 capfuls)
13) Finish topping off tank and dispose of waste water all before the 1 Hour mark (i.e., one hour from when I added the peroxide).
14) Add remainder of Flourish Excel dose (for a total of 5 capfuls, as instructed)
Everyone survived the immediate process. The old lady guppy was swimming at an angle within an hour of the end of the whole battle, and within 2 hours, she and my oldest male BN were dead. A total of 5 BNs were dead before I went to bed (say, 6 hours after the One Two Punch maneuver) and I woke up to the remaining 2 also floating. They all seemed to be extra-puffy, like they'd managed to swallow air, but I couldn't bring myself to necropsy any of them. I thought my differently-abled goldfish was also going to the great tank in the sky, since he seemed to be at a funny angle and resting more than usual on the substrate, but he survived the night and now looks fine. My betta, of course, is invincible, and while I was adding water, I did notice a few guppy fry that seemed none the worse for wear.
As far as plants go, some of my egeria looks melty, but still looks like the vast majority will survive. My wisteria, which had been the greatest victim of the algae, looks greener than ever before, and my java fern looks like it couldn't care less. A lot of my vals had been suffering before the 1-2 punch, since it was also a major host for the BBA and I'd cut most of the leaves in half in the past week, so while they look sad, they look no sadder than they did before the 1-2 attack.
I'll update on my plants, since I know anacharis and Vals are both reported to be sensitive to this double-punch. If I had it all to do over again, I definitely would have used less H2O2. I think anyone that attempts this technique should be prepared to NOT see anything during the 15 minutes of H2O2, and should keep in mind that any damage to fish might not show up even for several hours after the water change is complete. If I had been more patient and had more will-power, I wonder if my beloved BNs might have survived.
Sigh. Live and learn, I suppose.