I am running a 30 gal jungle aquascape with high lighting and CO2, but I have a dilemma... when I set up the the aquascape I only added a super tiny layer of eco-complete plant substrate in between some regular gravel and sand (with the sand on top). The substrate is only around 2 inches deep at the most and many of the plants I am using are heavy root feeders!

How can I add more plant substrate without mixing too much with the sand? Also, how long is plant substrate (eco-complete) useful to the plants? Must it be replaced eventually?

Thanks!

I would just add root tabs to feed your plants. Adding more Eco won't really do much for you. It is supposed to be good at storing nutrients rather than actually providing them.

Eco complete is great because it doesn't break down over time, once its nutrients have been used you just start to use root tabs and be on with it, I'd recommend at least 2 inches of eco for good results, I've used eco complete and capped it with iron sand for a smooth finish. I've come to terms that the eco will mix with the sand when I have to start adding root tabs but ill worry about that when it comes to it

As noted above, EC does not actually fertilize the plants. All the minerals it boasts about are actually locked up so tight the plants cannot get to them. Sort of like your dinner plate- you do not get your daily mineral needs because of your dinner plate. You have to add fertilizer to the tank the same way you put food on your plate.

On the other hand, if you need to add to the substrate for looks, or more root volume for the plants, you can do that.
Put the EC in a soda bottle, something with a narrow neck. Put the bottle in the tank and aim it toward the low spots. As the water enters the bottle and the substrate exits a lot of the debris will stay in the bottle. Probably a good idea to rinse it at least once before doing this, but I have seen some really impressive demonstrations with sand.