Valcanic pumice would probably work if you mix it with CHC - I would try 50/50. I use CHC and Stalite gravel in florida. Have used CHC and pearlte. I like the 1/2 stalite but also use 3/8 successfully. We dont have any rocks in Florida so whatever we use is shipped in.
In Hawaii those who don't use a mix of Orchiata (New Zealand Bark) with the CHC, use the 50/50 mix with volcanic cinder and CHC. Caution not to use the crumbly one ..generally red as it turns to dust in a short time. Use the one that stays in tact..Just a thought..