Help! Black spots and yellowing leaves

I'm doing a small indoor grow under 4 23 watt cfl pretty good ventilation in miracle grow soil it's my first Time trying to grow everything was going great until lower leaves started to yellow then black spots started to appear on leaves that we're yellowing and now have been showing up on even healthy leaves what can I do to help stop this

Can you post some pictures? Without pics its hard to tell you something.

How old are your plants? Are they in flower? Are you feeding other nutes? The more info the better.

Pics would be best...