hi,
just speaking from my own limited experience. after replacing my small external filter about 3 months ago in a 30 liter nano with a decent eheim, the water exited the lily pipe with such force that it nearly killed a narrow leaf microsorum on bogwood at the opposite end of the tank, and created a vortex scenario which did not penetrate to the substrate level, thus my p helferi and hc did not receive co2 and nutrients and virtually faded away. i replaced the lily pipes with a spray bar mounted on the rear pane at the top pointing towards the front. you can see all the plants moving in the flow now, and the aforementioned plants have made a complete recovery. also i have been able to turn the filter up to full power whereas before i was having to stifle it. you just have to keep the rear plants trimmed to keep them below the outlow, but they shouldn't realy be trailing anyway. i'm in the process of setting up a 300 liter aquarium and the spray bars are already in place!
cheers,
bazz!