My guess would be a swim bladder problem. If a fish can't maintain neutral buoyancy, it will either float to the surface or sink. Your fish sinks, and then it falls over on its side because comets, unlike fish that naturally have a small swim bladder, are not designed for benthic parking. If your fish always falls on the same side, then the upside probably has slightly higher buoyancy. When it swims, the fish keeps off the bottom using the power of its muscles. But because it's ill, it settles back to the bottom to rest again instead of treading water all day. Does this theory fit what you've seen?
The usual advice for buoyancy problems is to fast the fish for a few days and then feed it peas. This is based on the assumption that the problem isn't actually the swim bladder, but the an obstruction in the GI tract. However, comets are a lot less likely to have this problem than fancy goldfish. And it sounds like your fish isn't eating anyway.
I don't want to tell you how to treat your fish because it's not clear to me what's causing the problem, and an irrelevant treatment would just put more stress on your fish and perhaps interfere with its natural healing process. Nature is wise, hence if you've got no clue, it's best to let nature go on doing what it was doing. If I were you, I'd post on all the goldfish forums until I found someone who knew more about heavy comets. http://www.kokosgoldfish.invisionzone ... ase-diagnosis-treatments/
100mg/hr into 1500l is .067mg/l hr, or about 1/15th what the rule of thumb calls for. Of course, that rule assumes that you'll reach your ORP target of about 350mv, and the controller will then switch off the O3. As I said before, I'm no expert on these systems. But from what I've read, I think it's unlikely you'll reach dangerous levels with the Hailea. However, an ORP meter will let you see if the ozone is making any difference, and it's a is a great tool for understanding your water quality anyway.
I have not tried ozone, but I did do some reading on the subject. Basically, there are two types of ozone systems. One adds only a small amount of ozone, probably won't have a noticeable effect unless your water quality is fairly good to begin with, and can be left on 24/7. All of the affordable options are in this category.
The other type is hideously expensive, much more powerful, and has the potential to kill your fish. These systems have a built in ORP meter, and turn on and off automatically. If something goes wrong with this control system, the ozone may become concentrated enough to bleach the gills of your fish. Even if the meter is working, you'll probably want a waterfall or something similar to help gas off any remaining ozone before the water returns to the pond. These more powerful ozone generators are the ones claimed to keep the water sterile. Whether or not that is a good thing is debatable.
I think you're doing the right thing. Monitor the water quality carefully, and do a partial water change if anything gets too high. Aim to reduce the high reading to maybe half of what you consider acceptable. You want to keep the reading somewhat high to feed the bacteria, but not so high that it poisons your fish.
I've never used it, but some zeolite might help you keep the water quality up while your bacteria are catching up with the load.
In hindsight, it might have been a good idea to cycle the new media with ammonia before installing the filter. If you happen to own a time machine, go back and do that. Otherwise, just keep checking the water until the new filter kicks in.
If the net were tightly stretched a foot or two above the water, the herons couldn't get close enough to stab your fish.
An outdoor dog is one of the best deterrents. I've never heard of anyone loosing fish while a dog was on the job, though I suppose it's possible if the heron were bold and the dog were either lazy or fast asleep. And I suppose it's possible a dog might learn to fish, but I've never heard of that either.
Heron statues may help, but I think a humanoid scarecrow is more likely to be effective. With any passive scarecrow, you have to move the thing around or the herons will figure out it's just furniture. Also the boldest birds will land anyway, and when the scarecrow doesn't do anything, they'll go fishing. For this reason, a scarecrow that moves in the wind will probably work better than one that doesn't.
There are lots of other things that will work some of the time. People who have those motion-triggered sprinklers seem pretty satisfied, but that won't work for winter, and you'll probably get soaked a few times too. Trip wires will help if you can guard all approaches to the pond and there's no place to land inside the "fence", but these can be quite effective at tripping ponders too. I think a pet fencer would be more likely to work. Herons are cautious birds, and should give up on the pond entirely after a good jolt. I seriously doubt they'd persist through enough zaps to figure the thing out. But if you forget to turn it off when you work around the pond, you'll probably wish you'd used fishline instead. Then there's that traditional japanese gizmo. I forget what it's called, but water runs into it until the container reaches the tipping point, then a loud wood on wood whack and the sound of rushing water should scare most herons off. Of course, the noise may drive you or your neighbors to distraction as well. Shooting the bird with an air-soft gun (plastic BB's) is non-lethal and is said to scare them off for good, but this may be illegal where the law forbids harassment of herons, and you'll need some other defense when you're not watching.