Pics would help. Phosphate and nitrates being low doesnt mean anything if you have GHA taking it all up quickly.
Carbon dosing is quite easy but you will need a good skimmer and do it slow and consistently. A doser would really help. I recommend kamoer bluetooth doser, single head. You can use Nopox or other sources like vinegar, vodka. Time to do some research!
It's kind of hard to see, but this whole rock is covered with GHA. I've actually been looking into the kamoer doser!