My 40g tank is going through its cycle right now(day 11) and I have rust colored spots on the sand and glass, i think it is diatoms. Look at my 40g thread pics and see if it is similar.
Day 4 of cycle:
*Still haven't added the live rock yet, I will try to get some tomorrow or friday
Water Parameters: according to API test kit
Calcium: 480+
Ph: 8.2
Nitrates: 10- 20ppm
Ammonia: 0.25- 0.50ppm
Alkalinity:12