Maximum,
I would suggest that you should know that Herons are a protected species in Canada.
If you read on Heron feeding habits, you will see that about 60% of their diet is frogs and insects. Being opportunistic, they will once and a while pick one of your trout, as will king fishers, otters and minks. Mostly they will target minnows and shore dwelling creatures.
Once water warms up a bit, you should not see any trout in shallow water as this is going to be too warm for them. Plus trout are very afraid of these birds.
Loosing trout to herons is like loosing deer to coyotes, not fun but a fact of life.
Consider that loosing a few trouts to the herons is like paying interest on loan money... it cant be avoided... and killing all herons will not work as more will come!!
Learn to love the creatures, you'll see how regular and steady they are, they will always come at the same hours, do a walk, pick a few frogs then move on...
If you want to prevent them from coming, just install a fishing line about 3 feet off the ground that goes around the pond, it won't like to get tangled in it in case it needs to fly away. It will avoid it, and may abandon your pond. The fishing line, if well tighten wont be seen and will keep the pest away! It works for geese too!
For the aeration, I have clients who aerate their pond all winter and with the snow cover we get here, do not have super cooling problems. They mostly loose trout due to lack of aeration than too much aeration. Keep posting, its fun to hear about our frozen north that is finally thawing!
