if an anemone would let go prior to ripping, we wouldnt know of it. That is just not the case... While the animals recognize stress, they do not have muscular reflexes like ours and wont jump out of the way of a moving car or off a hot stove. they will die.
Try to remove a little water and add a little water. Keep the water level as close to the norm as possible. Instead of taking all 5 or 10 gallons or whatever out at once and replacing them all at once, do it in intervals.
An anemone in the top corner of the tank may be a sign of low flow or light in the tank. What is your turnover, lighting?