Please need some advise

Hello all and thanks for any help in advance.



I have just setup my 200l marine saltwater tank again after a year. The old rock and sand has been sat in tank with saltwater for this period

May be a bit long but will list what i have done and where im at.



I cleaned all old rock and sand with ro water properly

I then put rock and sand back

I made up the water and added to tank.



So i then obv needed to start the cycle process so i bought

from the fish shop a new piece of cured rock and say a cup full off sand to start the process.



Day one of cycle all ok ammonia up 0.50ppm nirite 0 nirates 15ppm

Calcuim 480ppm ph 7.6 and Ch 10dkh



BUT day 2 today i have come to my tank after beeing out for the day and one of the old live rock is BLACK i have checked it and i CAN scrap off the black. My reading are ammonia 0.32 nirite 0 and nirates 7ppm



I just want some advise on the black rock hope have provided enough info and thanks for any advise in advance:)