Lowering ph without tannins: alternative methods

[Feline]

I'm running a 40-gallon tank with a mix of low to neutral pH fish, and it's pretty heavily planted. The issue is my tap water pH is around 7.5, but I've got seiryu stone in there, which drives up both pH and kH. To manage this, I've been relying on Indian almond leaves, but the tannins are really limiting the light and stunting plant growth. I do get a brief window of better growth after a water change, before I add new leaves, but I'm looking for alternative methods. I've got some leftover fluval stratum that can help lower pH, but my tank's dirted and capped with sand, so I'm not sure how to incorporate the stratum without causing a mess and releasing all the soil into the water.