It can be a challenge to get total control of weeds. It's especially hard if the pond slope isn't just about right. Too shallow and you probably going to have issues with weeds. Also, if you've got runoff from heavily fertilized ground, that will also add to the weed population. Having a good grass stand around the pond to catch some of the runoff seems to help, and will keep the water cleaner as well.
In fact, I've reached the point of education through trial and error that I'll now accept some weeds as being healthy for a pond. This may be even more important if you are trying to have a healthy fish population. The year I killed off everything led to poor fishing the next year. In my research, I realized I had taken away a lot of habitat which is beneficial to a good fish pond.
If you are trying to control cattails and the like, the Roundup based products (I can't think of the specific brand names) are effective, but really they work best in the fall when the plant is trying to take nutrients to the roots. And at the recommended mix, won't hurt your fish.
If you are trying to control scum, you may need to add aeration. On my small pond, I put up a windmill aerator, and I never had to do anything else to that pond last year. No chemicals, no pulling weeds, no spraying-I'll save the cost of the windmill at that rate in about 3-4 years, and not have to lift a finger to do it!
Speaking of scum, chemicals like copper sulfate will kill the scum, but at the cost of wiping out the beneficial bacteria in the process. I'll never use CS again. Instead, I use a liquid beneficial bacteria that you can get at most farm supply stores and just pour it on the scum and leave it alone for a few days. Generally, that will take care of it. I've also sprayed it from my hand sprayer to get better coverage. If I don't have the liquid handy, I've found that good old Rid-X like you use for your septic system will do the same thing. I'll just broadcast it from the box, or use a hand held seed broadcaster. But watch the direction of the wind-don't ask how I know!
No matter what I do, I know that at times I'll still have more weeds than I care for, so I'm planning on building a weed rake for my tractor loader like I've been watching on other current threads. But that is a great problem to have-getting to spend time on the tractor!
Good luck!