Sounds like you have a copper problem. What chemicals have you used in your pool? If you post a whole set of numbers, FC,CC,TC,PH,ALK,CA,CYA, and tell us what you have used in your pool, we can help you better. I don't understand how you got a bunch of hairline cracks in the pool? Have you contacted the manufacturer? I have a fiberglass pool for 7 years, and the only cracks I have gotten in the gelcoat are on the top lip of the pool. I would contact the pool contractor, because it doesn't sound normal to me. Don't worry, you will be able to get rid of the stains.