Once you pump the water to the bio barrel, you are no longer "pumping" and your filter is
using gravity to flow and produce the waterfall. If you're unable to flow much it means your
filter probably isn't high enough over the level of the water in the planter/waterfall. How much difference
in height is there between the outlet of the bio filter and the water level in the planter/waterfall?
When you turn up the pump, does the water level also rise in the planter? If so it may be that
the waterfall outlet in the planter is too small. It's hard to tell how large that is.
My sieve gets little gnats hatching in it too. They make mosquito dunks that work really well. I use
them in my 10 gallon containers I have some lilies in. I've never used them in my sieve for the gnats...
not sure if it would work on them or not. Is the filter covered? I know when I put a cover on my sieve,
that's when the gnats appear. Very few of them show up if I leave the lid off.