Just a few questions first. What type of weeds do you have? About what percentage of the pond contains the weeds? What is the average depth of the pond/about how much is less than 4 ft? Is there alot of nutrient rich runoff entering your pond? There are 3 ways to eliminate the weeds currently in your pond and a couple of ways to reduce the chances of massive weed growth. They can be eliminated by physical means such as a rake or grabbing and pulling, you won't get them all, but you could thin them out. If the primary concern with the pond is fishing it'd be ideal to leave 10-20% of the existing grass, but there is always the chance of it spreading wild again without some maintanence. There are biological methods such as grass carp that could be placed in the pond. They are somewhat selective in there feeding but if they find what they like they could virtually descimate your weeds for years. The down side of that is little cover for future generations of fish. There is a book from the Illinois Natural History Survey that gives recommendations on grass carp stocking. The recommendations are for IL ponds, but there may not be a huge difference. The book is called "Controlling Aquatic Vegetaion with Triploid Grass Carp" and it's circular #57. The 3rd option is chemical applications. This time of year is probably the worst to try and kill all of your veg. due to poor oxygen levels. It'd be better to kill 1/2-1/3 every 2 weeks. You'll have to do some research to find out what will kill the types of weeds present in your pond. Ideally one would want to treat his pond earlier in the year when the water temps are in the 60's and the veg is first starting to emerge. This will decrease the amount of veg in your pond over the course of the year and save you money in chemicals. It'll also decrease your chances of a fish kill. There can be opportunities to prevent weeds before they start also. If you know how nutrients are entering your pond you could try and filter some of them out with a good wide stand of grass in areas where water runs into the pond. If yard fertilizers are entering the pond, try avoiding fertilizing on slopes that may directly enter the pond. Alot of times though ponds are nutrient rich just because of the soils and I don't think there is a lot to do about that. Another option could be aquatic dyes. These will act like a blanket over your pond decreasing the depth light can penetrate, which next to nutrients is a huge factor in weed growth. In the same breath though, adding a dye to decrease light COULD Possibly decrease the amounts of invertabrates in your pond which means less food for younger fish. (I don't know that for a fact, but I've seen that point argued and it made sense to me). Personally, in a pond I would use primarily for fishing I wouldn't use it. Back to your original question of killing fish or stunting growth...too many weeds could kill your fish if the weeds died around the same time and at the wrong time your oxygen levels could crash causing a fish kill. Chances are you'll probably be O.K. In the long run, year after year of thick veg. it could cause some stunting due to the vast amounts of hiding places for young fish eventually creating a large abundance of fish that couldn't be naturally thinned out by predators. Conversly though a small amount of veg in a pond can create cover for small fish in a beneficial way. Sorry this is so long, it's been a while since my last post and I felt like writting.