stress coat isn't compatable with skimmers IMO. Seachems prime will not effect skimming one bit. You say you did the water change not so recently, so, probably not that, unless you cleaned skimmer @ same time as water change, then, it would take longer for skimmer to overact. Some times back pressure on the return (submerged w/o air gap), or clogged air intake will cause this. What kind of skimmer (they all have their own lil problems) are you using? Some skimmers kick in long after you adjusted it, ETSS recommends 24 hour between adjustments, allowing time to moniter last one.