Thanks. Good point about the possibility of increased alkalinity. I have a large planter that's been set up a few years that I can check the ph. Soil is 50% peat and roughly 35% permatill and 15% perlite.
Several years ago I had plecostomus in a planted aquarium. I needed breeding caves for them and remember that clay that was 'over heated' ( I think that would be the term) in a kiln wouldn't change the water chemistry. Not sure if that's comparing apples to oranges since aquariums get routine water changes.
I've successfully used 50/50 peat perlite in pots. I'm planning on redoing a bog that made from a prefab pond (100 gallon I think) and I'm not sure perlite would be the best choice.