Let's see. Wow. Yeah, not QTing new fish usually bites you in the butt, happened to me last year, took forever to get rid of the parasite a baby BN pleco brought in because I couldn't figure out what it was...in fact still not sure if it was a parasite or some sort of nasty virus or bacteria he came in with, mixing an antibacterial, salt and an anti-parasitic seems to have thrashed it(I got desperate after a few months of UGH MY POOR FISH). x.x You can try that since it could be bacterial, or it could be a parasite at this point, melafix and General cure or another anti-parasitic, not sure your fish will like salt though. I also added some aquarium salt to their food, which they freakin loved and that could have done it.
Anyway, the only things -I- know of that causes a fishes scales to fall off like that is an infection, usually caused by stress, dirty water conditions(this is the MOST common one, but you seem to have it covered according to your parameters...). Parasites can sometimes cause damage to where infections happen under the scales and they fall off. Try treating the tank with an anti-parasitic medicine. Also keep in mind that melafix is best for smaller things, like a little damage or light infections...it's good stuff, but it has its limits and will be out-done by heavy infections, sometimes using salt with it can help with bigger issues...though most of your fish will not like that.
How is your Siamese algae eater? Often they will feed on other fish and this can cause the scales to rot off when the slime coat is damaged, it will eventually kill them as it opens them up to massive infections and parasites...basically they eat the protective coats of fish, and become addicted to it, which in the end of course, takes away the fishes ability to fight off said infections and parasites. At this point though...I doubt he's the culprit, they don't always go that way, but it is a possibility.
Anyway, the only things -I- know of that causes a fishes scales to fall off like that is an infection, usually caused by stress, dirty water conditions(this is the MOST common one, but you seem to have it covered according to your parameters...). Parasites can sometimes cause damage to where infections happen under the scales and they fall off. Try treating the tank with an anti-parasitic medicine. Also keep in mind that melafix is best for smaller things, like a little damage or light infections...it's good stuff, but it has its limits and will be out-done by heavy infections, sometimes using salt with it can help with bigger issues...though most of your fish will not like that.
How is your Siamese algae eater? Often they will feed on other fish and this can cause the scales to rot off when the slime coat is damaged, it will eventually kill them as it opens them up to massive infections and parasites...basically they eat the protective coats of fish, and become addicted to it, which in the end of course, takes away the fishes ability to fight off said infections and parasites. At this point though...I doubt he's the culprit, they don't always go that way, but it is a possibility.