Vivarium setup for large amphibians.

[Draco1264]

I know most of you deal with salamander's and newt's but I have a 50g long tank for my giant pixie frog. I treat his tank like a newt tank in the way everything safe for newt's he can have in his tank. I have the drainage layer of hydroballs, the exo terra mesh, LED lights along with a ceramic heat emitter and die mounted heat pad. My concern is the mix of substrates. I used coco fiber and worm castings mixed to start but it has lost most of its nutrients for the pothos over the past 4 months. I was wondering what mix of organic topsoil from garden magic red sedge peat to coco fiber to worm castings should be. The magic garden topsoil contains peat and sand only while the worm castings is pure worm poop and we all know about coir or coco fiber. Thank you for any input as this is my first time using topsoil in an animal enclosure.