I don't keep caridina yet but here's what I've read online:
Certain caridina species, including crystal red shrimp (CRS), prefer acidic pH and very low kH. Some caridina species don't need low pH (e.g. tiger shrimp), and some caridina species that originate from low pH environments can adapt to non-ideal parameters, but for the purposes of this question let's assume it's a high-grade CRS that's inbred and intolerant of less-than-ideal parameters: acidic pH and low kH.
To provide optimal conditions, caridina keepers will use RO water and add mineralizer to increase GH. RO water has a neutral pH, so we need to add aquasoil to reduce the pH.
Since caridina prefer low kH, the aquarium is subject to pH swings. This is why we need some active substrate to keep the pH at ideal levels.
The TLDR of what I've read online is that active substrate reduces the pH to a certain level (6.0-ish) and, more importantly, it keeps the pH stable at that level even when the aquarium has low kH.
What I haven't been able to identify is this: what's the magic sauce in active substrate that provides this alleged buffering capacity? Can this be reproduced without using expensive aquasoil?