Very strong magnets. There is no issue holding up on my 5/8" thick glass and honestly you are unlikely to find any other frag racks that have magnets which can hold up onto 5/8" or 3/4" glass. The only cons is the frag holes are a bit small at 7/16" dia. and will not fit Oceans Wonders or Boston Aqua Farms frag plugs which have stems also at 7/16" dia. However you can easily enlarge the holes using a reamer or buy Coral Farmer frag plugs which have stems of 1/2" dia.