If you are going to use LECA I'd suggest doing your substrate like this

LECA > SCREEN > soil

The important thing is putting a screen between your LECA and soil. In the long run it will get the substrate out of the LECA the best.

About the wicking water, with LECA i think you will need to pay way more attention to your soils soggy ness with LECA because as mentioned it wicks the water up into the soil so to keep this at bay you will probably be siphoning more than you are use to. With a water feature i think eggcrate is the way to go for reasons like this

If you are housing the water pump inside the tank, it will be more of a pain to dig it out of the LECA if you need to get to it. Using eggcrate can allow you to create a sectioned off area to house your pump for easy maintenance and being able to access it without disturbing the Viv. Sure you can stick your water pump down in the LECA but i think having it under a eggcrate FB as long as you make it accessible is the way to go and you would thank yourself later

Hello, new here. It's a little off of the original topic but, I do have a suggestion for gaining access to a pump for a waterfall/stream feature. I haven't constructed it yet but my plans for my 29 gal fish tank is to do a 3"x3" section in the corner, from bottom to top, of egg crate which I will cover with great stuff for my background. The sectioned off area will allow me to gain access to my pump by just sticking my hand/arm down there. A tube from the pump into a small plumbing elbow that will stick through the G/S to create my stream.

As far as using just LECA, my first tank that I made about a month ago, I only mist 2-3 times a week and I can see the moisture in the LECA and I even have an egg crate F/B under it. If you're going to have pure water mixed with the LECA, #1 it will expand, #2 what was mentioned before it will "wick up" meaning it will act like an oil lamp. The oil is in the bottom that goes up the wick. In my short time of doing this, I believe you'll be better off using the egg crate.