False bottom or LECA drainage layer in a 36 inch tall vivarium on a budget Substrate

False bottom, for exactly the reason you flagged.

Two bags of LECA plus the water it holds plus three inches of wet substrate on top adds up fast, and an ordinary cabinet under a tall tank is already carrying a lot. Egg crate on PVC pipe legs with a layer of window screen over the top weighs almost nothing, gives you more usable water volume in less height, and costs a third of what the LECA does.

Build notes from doing several:

• cut the egg crate slightly undersize so it does not bind against the silicone in the corners
• use short lengths of PVC pipe as legs, one every 15cm or so, which is far more than you need but the crate sags under wet substrate otherwise
• fine window screen over the top, not weed fabric, which clogs
• put a length of pipe vertically in one back corner down to the false bottom so you can drop a siphon or a turkey baster in without digging up the tank

That last one is the thing people forget and regret. In a 36 inch tall tank you cannot easily reach the bottom to drain it once it is planted.

Whatever you choose, do the substrate barrier properly. Fine window screen or a layer of fibreglass mesh between the drainage layer and the substrate, cut to the exact footprint with no gaps at the edges. Substrate washing down into the drainage layer is the failure mode that eventually forces a teardown regardless of which method you picked.

I will make the case for LECA, because it is not just about weight.

It is faster to install, there is no cutting, and the irregular surface holds humidity and hosts microfauna in a way a sealed void does not. In a tall tank where you will be misting heavily, that buffer is worth something. It also cannot sag, and a badly supported egg crate under six inches of wet substrate does sag.

That said, if your cabinet is ordinary and the tank is tall, weight is a real argument and I would probably still go false bottom in your specific case. Just do not treat LECA as the amateur option, it is a genuine tradeoff.

One more consideration for a tall build: get your background and any mounting done before the drainage layer goes in. Working in a 36 inch tall tank with three inches of wet substrate already at the bottom is miserable, and anything you drop lands somewhere you cannot easily reach.

Check the cabinet itself while you are planning. A full 18x18x36 with substrate, water and hardscape is heavy enough that a flat pack cabinet with a particleboard top can bow over a year. A piece of plywood cut to the footprint and laid on top spreads the load and costs almost nothing.