I changed the elbow to a Tee and it works. Updated pictures and info are here
. Now let's see if it actually does anything for my nitrates or just looks nice.
One thing if I were to do this again, I would not only use a Tee but also make the output a larger diameter than the input (as Nil suggested). Even when the ouput is flowing, it can't keep up unless I reduce the powerhead's flow rate to pretty low. I wanted a low flow rate anyway, so that's not really a problem, just a puzzle. I don't really understand why this is - maybe somebody could explain the physics to me.
BTW, it turned out the Tee did have a flange just like the elbow (the picture on the website was wrong), so no need to add a washer.
Thanks for all the comments and helpful suggestions.