I've used up to 4% H2O2 in my tanks with shrimp without a problem. I don't even do a water change after the treatments since H2O2 naturally breaks down to H2O and O (water and oxygen). So I question whether the H2O2 was the cause of the deaths.
For starters, whenever you do a water change, you should be using Seachem Prime or some other water conditioner to remove chlorine and chloramine from the water. If you aren't, then that would be a potential cause of shrimp death.
Also, if you have used bleach on something without getting it all back out before putting it in the tank, then that could kill the shrimp.
Another possibility is that the attempts to kill the algae has altered the balance of good bacteria. Sometimes what happens is the dead algae decomposes, overwhelming the amount of beneficial bacteria in the tank. Without enough bacteria in the tank to handle the increased demands of the dead algae, not all the decomposing algae gets converted into nitrite and nitrate, causing a spike in ammonia or nitrite. The tank will usually compensate for the spike pretty fast, but sensitive shrimp such as CRS may be killed in the process.
Further, while cleaning the algae, it's possible to disturb a substantial amount of beneficial bacteria which also causes an imbalance, throwing the tank into a mini-cycle, harming sensitive shrimp in the process.
It is important to look at everything that happened in order to determine the cause of the shrimp deaths and, in turn, to determine a possible remedy. If the H2O2 wasn't the actual cause, it might explain why performing water changes wasn't helping.
Vicki —Rena Filstar pimp #142 (four XP4s/three XP2s/one XP1) • Eheim pimp #301 (Pro II 2128) • Victor pimp #27 (VTS-253B-320)
• 90g - Journal
Pelvicachromis taeniatus 'Moliwe' ——
• 75g - Journal
Pelvicachromis pulcher 'Lagos Red' Better Pics 8-24
• 29g - Journal
Pelvicachromis pulcher 'unknown' —--
• 29g - Pelvicachromis taeniatus 'Moliwe'
• 5g - RCS colony ——————————————————
• 2.5g - Journal