How do you control the outflow pressure(decrease it ) on one of these -
I
hooked it up to a 20 gal and the extreme turbulence killed a couple fish

You should be able to get an inexpensive AC potentiometer from an
electrical
supply house (or perhaps even Radio Shack) and wire it into the filter's
power cord. If you want lower flow, just reduce the amount of power the
filter can draw.


Do that and the motor will die. It will try to draw too much current and
burn out...