black stain

[jemae]

hi, been running my new inground pool for the past 3 weeks. the question i like to know:

1. I start seeing light black stain inside my skimmer, clean the cartridge filter today, the filter also look like infected by this light black stain. the black stain can be easily clean.

2. how often do you have to clean the cartridge filter? usage about 2 hours a day at the most, around 4 people. pressure reads about 100 to 120 kpa, after cleaning, pressure drop to about 70 kpa, pump 1 hp running 9 hours per day

pool info, 15 000 gallon inground,
total chlorine 2
free chlorine 1
total hardness 100
total alkalinity 100
ph 7.2

added chlorine 75% strength about 240 grams, shock up to 600 grams once a week, water quality clear.

thanks