Thanks for the info-but red line doesn't really 'disappear' below 30ft(it just doesn't look red anymore). As spearfishermen know-when you shoot a fish you see the blood as a dark greenish color-if red was truely 'invisible' the blood would be 'invisible'-try taking some red line down next dive and see if it 'disappears'----just wanted to clarify some common misperceptions-red line may work better-but not b/c it's invisible