Welcome! I have gained SO much here its crazy. Wish I found it BEFORE we built our pond...would've been much better.
Anyways, I would start be identifying the weed in question.
Here is a link to an A&M site that would help alot. Might try posting a pic of it also for the experts here.
Texas A&M Aquatic Plant ID Mechanical removal would be great if you could get most of it. There are sprays that can do a great job...but it is summer and whenever you kill any aquatic plant they consume mass o2 as they decay. You would want to do small portions if you sprayed. Not over 1/4 or so at a time. You might find by identifying the plant that it can be beneficial in controlled amounts...good place for the little guys to hide out also.
Best way to gauge what you have in the pond is to seine it. There are alot of posts on here about it. You would probably want to get most of the weeds under control first. Seining won't tell you exactly but can give you an idea and an avg.
Hope this helps, the experts will chime in soon I'm sure.
Have any pics of the pond? WE LOVE PICS!! And again WELCOME!
