Took a second look and a video and I think it does look like the foxface and trigger are breathing heavy. I don't see anything on the fish so my guess is internal parasites or maybe flukes?
Given the previous fish losses and the triggerfish's rapid breathing, this is an external parasitic problem (possibly bacterial, but unlikely). Water quality and diet is important of course, but they have to be REALLY bad to cause fish death and broad symptoms across all fish in a system. For example, nitrate is often > 300 ppm in some large aquariums back in the day and the fish were fine. Same thing with nutrition - you have to feed fish REALLY poorly to cause a group of them to outright die.
The lionfish is in a pretty bad way, and once fish loss has begun, it is often not possible to stop the issue soon enough to prevent further losses.
I don't think the issue is a protozoan - it isn't ich and velvet/Amyloodinium kills fish so fast that you'd have no fish left by now.
Flukes cause rapid breathing, cloudy eyes and deaths spread out over weeks. Some fish may not be affected as flukes have some specificity as to their hosts.
I think you should consider treating for flukes. A freshwater dip can provide temporary relief, and with some species of fluke, you can see them in the dip afterwards, confirming the diagnosis. However, I don't think the lionfish would survive a dip.
The two treatments for flukes are hyposalinity (no invertebrates in the tank of course). I worry about that method here because your tank has a lot of organics in it, mulm on the bottom, etc. When giving a tank like that a hypo treatment, there is often a die-off of all the critters living in the tank and that fouls the water.
Praziquantel is an option, but you would need to do 3 treatments, as this fluke is no doubt one of the egg laying species and you need to break the life cycle as the prazi does not kill the flukes eggs.
If you decide to do one of these, let me know and I can walk you through it.....
Jay
