i encounter this before. its growing especially behind the wavemaker. i did nothing to it, just keep scrub it off. after a week or two, it doesn't show up anymore.
could it be lack of iodide/iodine?
my mushroom bleach till almost transparent. i didn't change the lighting hours, but i added kent's marine lugol's solutions. and it's red color came back brightly in days.
i read article saying - Carbon can only absorb so much and after that it may simply become a nitrate bed instead.
i wash the carbon (aquapharm black gold) once every month. and replace every 3 months.
dont buy those supermarket plastic bottles lei... so unfriendly for our environment...
i brought 2 carton before. after throwing 24bottles, i felt so bad..
dont flame me..
maybe i kiasu, i have roughly the same size as yours. my return pump was 6000lph, now upgrade to 10,000lph. i think just nice
i using tesino - about 250