Take it out & clean the suction cups with bit vinegar. Rinse well & make sure they are dry. Do a water change down to the level where you want to put this. Scrape the area you want to place with old cc or grocery card & wipe with paper towels until its clean & dry. Stick your powerhead back on. You may have to breathe on the suction cups if they dont stick the first time. If they still dont stick, try placing something warm on the glass to warm it a little bit (towel out of dryer or micro) before trying to stick it again. If the glass is too cold (or slimy/not clean), they will never stick. Hope this works for you!