I had Eco Complete for awhile under Flourite, and I honestly wasn't impressed by it at all. Plus for me it was a pain in the butt to plant in, but that's just me :p
I'd also like to caution you with capping Eco Complete with sand- because sand is generally much finer than the EC, it can fall through the gaps and 'push' the EC on top of it, eventually reversing itself in a way. I had this happen to me, and it was really annoying. I've heard of people doing it successfully but that wasn't my personal experience.
I think that Flourite Black Sand is just crushed up Flourite chips, so maybe that's why I have good growth with it, in tandem with my slight over fertilizing.
You could also go the route of a NPT (soil based tank), although I'm not sure if that's safe if the tank already has fish in it...soil has to stabilize before fish can be added, from what I've heard.
Anyways, sorry for the ramble! Good luck with whatever you choose to do, and I hope to see pictures! :)