Maybe your filterpad is too dirty or in someother way to constricted so water cant pass through. If your filter pad is the slide in type, like those found on Whisper and Top Fin brands, the water wont go through very well if the pad is too dirty. And with the way the filter slides in it can act like a dam and prevent water from crossing. The water will look for the easiest path and likely go back down the intake tube side of the filter. Thats why I like the Aquaclear brand hob
because the pads at the bottom of the filter. Plus you can put more media in them.
80 gallon FW
:3 clown loaches, 2 angels, bloodfin tetras and 1 albino BN pleco,1 Farlowella.
46 gallon FW
: kuhli loaches, otos, black neon and black phantom tetras, 3 pitbull plecos.
50 gallon SW
: 1 green spotted puffer