Hi Elle, I think I've gone to about 7cm, maybe deeper, I can't remember exactly. Don't think I did anything different, I usually just use a 1:1 mix of aquatic compost and moss peat with added grit for structure. It worked well with big root feeders like crypts and vallis. Plant roots are very leaky structures and will oxygenate the rhizosphere over time.
I guess deeper substrate also has a greater redox gradient and provides a more varied or stratified environment and therefore perhaps greater microorganism diversity and abundance. Which in turn could mean increased denitrification, breakdown of organic matter, and release of CO2 and nutrients and an increase in nutrient solubility.