I didn't have any problems dosing everything (SeaChem) including Excel, Except for P from one clear 1 gal jug. This time though I painted it with Krylon Fusion (Plastic Enamel). I put masking tape over my marks, painted the jug and then removed the masking tape--I can see the level in the jug and the amount of light that gets in should be negligible.....
2 coats of Black for light followed by 2 coats of Hunter Green for Decor....