I'd probably agree it could be staining, just thought I'd ask - no disrespect etc meant!
🙂
You must get through quite some fertiliser with a tank that size so spend a while mixing those small containers up - thought about bigger containers and a simple doser hidden away to make things easier? They do say not to keep mixed ferts in the light, quite the difference this makes I'm unsure
The driftwood was only a suggestion but it seems you've been that route before.
I guess Discus and the inhabitants that might help you stay on top of part of the crud issue are not compatible.
A method a lot of people seem to have take up now is using a turkey baster to blast the crud out of the nooks and crannies straight into the path of your siphon at water changes.
😉
People do plant in gravel; things like root tabs or even choosing plants that rely on water for their fertilisation.
Big (or small) pieces of stone with certain plant types can replace the wood and just have a minimal layer of gravel/sand.
If I'm honest I'm not confident enough to detail but many on here are if you want to avoid the aqua soil and go down the gravel route.
Flooding the thread with useless information, sorry!