Wow TheClark, either you have a lot of phosphates or low nitrates, assuming bacteria consumes nitrates to phosphates at 15:1.
I really wish I tested the tank prior to starting carbon dosing but oh well. I really should be testing anyway, mainly for my curiosity, but I just made an online order and won't need another one for months.
Indeed, I have a monumental phosphate leaching rock battle on my hands. Vinegar dosing was a huge success though with the nitrates. Many would highly recommend test kits while carbon dosing especially in the beginning when dialing it in.
Good luck!