Prevention is an imperfect discipline. Animals adapt to our methods and materials and workmanship break and decay over time. Monitoring and maintenance are necessary in any prevention program. Acting quickly before a problem gets out of hand will save you a lot of trouble down the road.
If you are looking to mitigate a problem that is already occurring please visit the solutions links in the menu bar. If you aren't sure what the cause of the problem is then visit the animal damage identification section. NEVER, NEVER, NEVER secure a vent or hole unless you are absolutely sure the hole or vent is no longer being used.
Tips for Protecting your fish from raccoons
Don't waste your time with repellents, lights, sounds (audible or not) or odors.
Use native fish whenever possible as they are harder for raccoons to see.
Grow Sedum or other spongy type plant around as much of the pond as possible. This plant makes it difficult for the raccoon to feel he has a secure place to stand on and can disrupt his balance as he reaches to scoop fish.
Install deeper ponds to provide more security as they are less likely for a raccoon to wade in. However, the surface of the pond needs to be balanced with depth so that the fish obtain sufficient oxygen.
Provide underwater structure so that the fish have more places to hide if the raccoon misses on the first pass.
Consider fencing. There is a non-electric fence called "The PowerKage". It is manufactured by Kinetrics. The fence employs a smooth surface that prevents the raccoons from climbing by prohibiting their ability to obtain the necessary footholds required to scale over.
Tips for Protecting your fish from birds