Turn off you ph and make a thick paste of kalk
, or inject boiling water, joes juice or lemon juice. If you have 1 large one, you probably have many many more small ones
I use The Majano Wand once a month to zap 3-4 small ones, usually only in my sump or overflows. I very seldom find them in the main, even though i look at night all the time. Hum bee curious if my Black spiny urchin has anything to do with than? The are not a big deal if you deal with asap, they can and will overtake your tank if left unattended.