Do a google search for Secchi disk. Learn how to use it. I believe plants can utilize sunlight to grow that penetrates 2x the reading that you get.

Also search on here for what dye does the the bottom of the food chain. It's something that I'd be concerned about if I had a pond and I wasn't feeding the fish on a daily basis with good fish food.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).