Chaz gave you some good advice. The PFBC would be able to help you. Also, ODNR (Ohio) used to have a section on their website for pond management, but the site changed, so I can’t give you a direct link.
If you are experiencing a fish kill, that means you have more fish than the pond can handle during it’s worst days. Of course the algae die-off is probably causing most of the problems.
The right kind of aeration doesn't raise the temperature significantly. The best aeration is with a bubbler, not a fountain. Fountains only oxygenate the surface, and do tend to heat the water. They help, but aren’t really that effective. They just look nice. But an air pump with a diffuser aerates the whole pond and doesn't heat the water. Aeration can cure a lot of problems, and can at least reduce the amount of fish kill.
Careful with the algaecide or herbicide. They typically don't directly harm the fish, but do harm the hatching fish as well as some invertebrates. This is true with ones that have lost of copper, like copper sulfate. Also, don't treat the whole pond all at once, unless you want most of the fish to die. The decay of this stuff eats up all the O2. and don’t treat duing the heat of summer when the water holds less O2.
Duck weed is a tough one. I have a little bit, but not enough to be much of a problem. Grass Carp really don’t help all that much. They will eat it, but only if there is nothing else. My pond is very fertile, yet I don’t have a significant duck weed problem. I think most of it comes from my neighbor's pond which occasionally overflows and runs across the road into my pond during heavy rains. They have a big duck weed problem, I don't.
I used to have a big problem with filamentous algae. I used copper sulfate sparingly the first year, and it helped, but I don't like using it. I also threw in a couple grass carp, but haven't seen them since. They could still be there, but I would think I would have seen them. The second year I bought some stuff that I thing is called Cutrine Plus. It works, and doesn't seem to hurt anything. Just make sure you don't treat the whole pond all at once. It costs about 35 bucks a gallon, and you dilute it in water in your pump sprayer and spray it right on the mats of algae.
I haven't had to use any chemicals the past couple years now which tells me I lucked out and now have a good balance. What I did was add channel catfish. When I pulled some of the algae and looked at it, I could see it was loaded with fresh water shrimp and bugs. I think the catfish are eating it to get the protein. In any case, I still have a little bit of this algae, but not enough to warrant any chemicals. I also surface feed the fish, and I think they end up eating some of the duckweed and algae. That is just a guess though.
Another thing you might consider is using some kind of dye in the water. A friend uses it with some success. The filamentous algae that he does get, he rakes it out. If you have water flowing out of your pond, this is probably not a good option since it will not last as long. The idea is to add shade which doesn’t allow the algae to grow (filamentous algae actually grows on the bottom and breaks lose.) I’ve also added some lily pads. This will probably end up being a mistake because it is spreading fast. Another thing to control.
Right now my biggest problem is cattails. They aren’t really a problem, they just look ugly, and are hard to keep under control. Also, there is an exotic species of cattail called narrow leaved cattail, and I am pretty sure I have some of that (which I highly desire to get rid of).
Trees provide shade, unfortunately, they also provide dead leaves. I have a couple willow trees starting, and I am going to let them go for awhile and see of they help with the cattails.
