Seamonkey,,,what brand of filter ya got there? if i know, mebbe i can help you better,,,in the mean time , like everyone has said:
bottom line , ya got a seal leak , or a pinhole somewhere,,
o-rings /gasket,,on o-rings, remove, and clean off well, apply i lil bit of silicone grease,Scuba shops carry it,,wipe of the excess, and clean the mating surfaces also,,,
on most hoses, check around where the hose goes on for a tight fit,,if it's loose,and can spare it, cut off a lil of the end and re-attach,,use hot water on the end b4 you put it back on,and when it cools , it'll make a better seal the just forcing it on.
another place to check,,,the "nub" that the hose(s) attach to on the filter, check for a crack along the nub area, flex the nub a bit ,while it's running, or when it's unplugged ,use a flashlight or good light ,and look for cracks,,you might see it better when you do a lil flex on it(i've had a few of those over the years) if it's not a bad crack , a lil bit of siliconesealer
should fix it up.
my experience is with Magnum330/350's and Fluval's(20 or so yrs)
and other filters pretty much are the same thing in theory,,
good luck, let us know what you find,,,