How can we keep our sea anemone from getting caught in the filter?

We are building a reef tank for our son who wants to be a marine biologist. Yesterday we added an anemone (the kind favored by clown fish), a fire shrip and some coral and . The anemone was all over the tank! Last night, I went to check the tank, and the anemone was stuck in the filter. I had to turn it off go get it free. Some of the tentacles are bent up and although it has been moving, it has not traveled anywhere else in the tank. How can we avoid this? It is a 29 gallon tank.