alford what a nice coral! Other than some transient local irritation monti's are not a sensitive. I get mine wet with 35% at times and it doesnt even phase it.
I would not treat that underwater, can you lift out the frag or drain down to the level of that algae? Id use a wet q tip to apply the peroxide lightly to the strands of algae, then put it back in tank. quick easy kill
the benthic organism you pictured sure looks like two brittle star legs...the micro ones
Here is day 3, significant improvements. I suspect that my snails were at the algae during the night. My observation is in line with others, such that algae, once "cooked" with H2O2, seems to be more appetizing to snails...
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And remember, it used to look like this:
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