I can tell you 100% certainty the problem is
very likely your pool has too high CYA and you don't have enough chlorine. As a former Frog user, you described my prior situation exactly. Read
this article here.
Here is my pool, 2 years ago, not long after I removed the Frog assembly from my system....
http://www.troublefreepool.com/post5558.html#p5558
These stains, that I had for 3 seasons, dissappeared from my pool within 3 months of switching to BBB, and have not returned.
Here's what you need to do:
1. Order one of the test kits
discussed here. Order them online, unless you can find the K-2006 in a pool store (rarely have them - accept no substitute; TF100 only sold online and a better deal for the money IMHO). Having your own test kit is gonna save you a TON of money and a TON of frustration and headaches.
2. Post a full set of test results, obtained from a Pool Store. DON'T BUY anything from them....just accept the printout and say you "need to think about it".
This will get you started.
(In the meantime, create a signature. Go to User Control Panel, then Profile, then Edit Signature. See our sigs for examples of the info we need.)
