ok, another dirty MS Paint job.. but what the picture is showing is a setup similar to mine.
Right back corner, replace the elbo with a T, on the top part of the T put a Slip-Barb adapter, get black hose the diameter you need, hopefully the loc-line and the barb are close enough for this to work with one short piece of black hose. What I would do, because you really don't need the Y splitter/loc-line return since your going to have the MP20, is I would remove the Y splitter and have the entire return coming out one loc-line piece. This would provide the flow to make the floating reef actually work for you more than just as a rock holder. I would drill 5 holes at the spots I put in the picture and drill them at an angle you think will work best. Start with 5 small holes, but big enough so grains of sand wont get stuck in the holes, and if they are like little jets then you can drill a hole at a time after that until the flow is just right. You want it to just be enough to keep the water moving upwards so stuff doesn't sink to the sand.
This is just brainstorming on my end.. don't feel any obligation to actually do this if you don't think it is what you want.. this is YOUR tank, so do what you think is best.