Sorry to read this, losing fish always sucks - especially when you have no idea what killed them.
You might try calling a university in the area. Sometimes they are willing to do studies, a biopsy, necropsy, etc. for educational purposes for little or no cost. Similar to the beauty schools that do spa treatments, etc. for 1/4 the normal cost....
Not that this is really going to help you now, but as far as the dormant theory goes - I think that is a real possibility. I am a believer that clown loaches (any wild caught species really) naturally carries parasites, etc. that it has picked up in the wild and those stay dormant until the fish's immune system is weakened. Then the parasite/infection/whatever is able to take hold and start destroying the fish, eventually bringing on secondary infections and resulting in death. These parasites, etc. can also be passed on to other tankmates, so even a tank-bred specimen can become infected if it has had contact with a carrier.
This theory does explain (again, example

why clown loaches can seem perfectly healthy in an aquarium and then succumb to "skinny disease", even if no new inhabitants were introduced. Also would explain why the new imports are more prone to have it, stress of being shipped, poor water quality, etc - some pull out of it and get purchased at the LFS, only to succumb later on in a healthy tank if immune system is weakened....
Prazi only treats certain kinds of internal parasites, and a UV sterilizer wouldn't help with them at all since they do not reside in the water column (I know you know this, just stating) - so the use of another medication (as previously stated) such as Levamisole, Metronidazole, etc. could be helpful. Followed with a broad spectrum antibiotic for secondary infections such as Maracyn I & II. I have never kept rays, so how this medication would affect them, if it is even safe to use with them, I have no clue. Just trying to help, adding to the list of possible causes/treatments.
I am truly sorry for your losses.