sorry to be more specific, i boiled rodi water in a glass cup. sucked some out with a coral feeder (thin turkey baster) and blasted it very slowly directly onto the hydroids while still in the tank. i squirted it like 1mm away, practically touching and sometimes touching. this way not too much hot water is spewing everywhere and very concentrated. the ones that died (or seemed to have died so far) are completely white. i didn't kill off anything else either it seems. i'll try it again, but i think i killed off 1/3 of them.
i made sure nothing else was within 8-10 inches from it. i really only have a jawfish and he stays away pretty far in his burrow. my hydroids are close to the top of the tank. snails i just moved out of the way.