So I tried this on my spec. I used spare parts from old filters. I used some clear tubing I had lying around. I used and elbow to attach it to the filters output. I ran it down straight and coiled the tubing in a circle on the bottom. I drilled holes into the tubing about 1/2 inch apart. The I added the gravel and stones. It has been running for a couple of weeks now. My Betta is very happy with the low flow and the tank remains clean. I wish I would have taken pics before adding the gravel but you can see the tubing running straight down and a happy Betta
That looks great. I was going to suggest what you did, use some old filter parts for a tight elbow. They are hard to find at a hardware store that are as small, especially with a tight radius.
Not being familiar with the sump part of the spec (seen a lot of them, never owned one), would it be possible to turn the pump sideways so you could run the tubing behind the back plastic? That would be really clean looking.