What are magmounts you ask? Simple… They are magnets covered in a waterproof coating to attach/mount rocks, corals, powerheads, etc in your aquarium. The magnets come in 2 sizes… smalls (about 10mm in diameter x4) are geared towards the aquariaist who want to attach corals to their existing rock structure(one mag on the coral or rock rubble while the other on the structure). The large(about 20mm x2) are for mounting corals/rocks/frags/powerheads though a pane of glass. One mag on the outside of the aquarium while the other to the coral/frag/rock on the inside.