cajinfry,
I went back and read your original post that said you were given the tank -- this might be stretching but by any chance do you know or can you find out if the previous owner used any copper based and/or algicide products in the tank at any point? Perhaps get a list of meds s/he'd put in while they had it?
Also, why did they get rid of the tank? Did they have any unexplained deaths?
Roan