the guy in the LFS is an ass for saying that! Should really try and tie them down soon as they wont saty in one place for long. Any stone (aquarium safe) or small piece of wood will do.
u can see in the bottom pic that ive used some slate to hold them on the bottom,been ok sofar.
ive been usein excel now for 4 days now and wow ive got plant growth in both tanks,
1 off my twisted valis has sprouted 2 new plants and the thin plants on the left have gone mad..at least 5 leaves are at the top off the water line..