Intro: <Dramatic horror movie music plays ominously>


Sorry to be so dramatic, but I absolutely REFUSE to go through all this again.
My pool is looking GORGEOUS and I have been testing my water DAILY (like a good girl).
My Stats:
Water Temp: 76
pH: 7.4
FC: 3
TC: 3
TA: 120
CYA: 40'ish
12,000 gal. IG, Gunite ~4.5 years old; Cartridge filter
Using only BLEACH now to chlorinate

I HAD to finally add some water to my pool -- have not added any since start-up and it has been so hot and dry here (well, until YESTERDAY). Anyway, I added about an inch of fresh water to my pool and guess what?
Yesterday I start noticing 3 or 4 small areas where the stain is trying to come back. This has GOT to be the water source! My pool walls/floor were *pristine* before I added the fresh water.
So, I have another bottle of Metal Out on hand and I'm wondering if it's ok to do the maintenance dose of 16 oz./10,000 gal every time I need to adde fresh water (actually, the bottle says to add this every week during the swimming season, but that seemed a little excessive to me).
What do you think?
Pool chem is great; water is sparkling clear. I do NOT want to go through the horrors of that Easter-Egg-thread again!
But I don't want to mess up my pool, either, now that I've finally gotten everything under control.
Please advise!