Welcome to the forum from your neighbor to the north!
First things first... Find out what you have. In no particular order:
1. Aerator, what compressor do you have? I assume it is a bottom diffuser and not a floating fountain? Help us out with make model or HP and then do you have 1 weighted line or more, going to how many diffusers? Have you pulled up the diffusers and cleaned them off? Might be a good time to check on your aeration system as it is going to be a key part of your future success.
2. While in the row boat pulling the diffuser up, take a weighted rope and either mark off feet ahead of time or sound the depth and measure rope later. The pond WAS 15 feet deep 25 years ago. How deep is it now? What is on the bottom? This will help you decide how much muck, leaves, etc you might be dealing with. The diffuser might also be buried in the muck by now.
Try to get a rough idea of the layout of the bottom, any ridges, humps, holes, large shallow areas etc. This helps you plan for the future.
3. Get a minnow trap of various sizes and size openings (or get several traps and snip open the standard openings to a bit larger to sample panfish as well as minnows if that is legal in your state) This will tell you petty quickly if you have sunfish, bluegill, green sunfish, perch and what age classes, large mouth bass young, and what forage you might have (minnows, crayfish, shiners etc)
4. If you are surprised by what you find then think of ways that fish can get in the pond during high water events and consider what your future plan is to prevent fish from getting in or out of your pond in the future.
5. Ok, one more thing while you are learning. Get a 36" aluminum ruler from your big box or hardware store and screw it to a stake or pvc pipe and sink it in the shallows. This way you can get an exact idea of how your water level fluctuates. Losing a little per week is normal with evaporation and trees sucking water from the pond. Losing an inch a day means you plan for finding a leak or providing means to fill the pond when needed ( a big well).
Sorry to give you so much homework on your first day here
You came to the right place! Many here can help.
Post pictures if you can, that helps the most.
