I'm thinking it's columnaris, a bacterial infection. You say the fin damage is mostly toward the rear fins, and a sure sign of columnaris is "fuzz" or lesions around the mouth. But the lady also says all the fish are dying, which may suggest columnaris. I had a fish once with a particularly bad case. The only symptom he had was the "saddle back" wound where his dorsal fin rotted away.
The article has some good treatment suggestions. I would personally stay away from copper and use an antibiotic instead to treat. It may be hard if the tank isn't at your house, but you could put the fish in the QT and use antibiotics.
Columnaris, like other bacterial infections, are probably brought on by poor water conditions or another type of stress. Like Rich says, water changes will help a lot too. Do you use 100% RO
water in the 5 gallons of water that you make for her? That's fine for a SW
tank since your salt mix adds back in the necessary electolytes, but in a FW
tank, tap water is better. If the tap is not suitable for some reason (well water that's high in nitrates, for example) you can mix half tap and half RO
. Start off slowly by mixing a 5 gallon jug with half tap and half RO
and have her do water changes with that mix. That will acclimate the fish slowly. 5 gallons is not much for a 55 gallon tank, so see if she can do a 5 gallon change twice a week to try to improve the water conditions.
The article suggests aquarium salt (not reef salt) can be added. That is a good procedure for a sick tank, but for this tank, if it is using all RO
water, I would be careful - the fish aren't used to the electrolytes that the salt will provide.
Before you get started with medicine, I would verify the water parameters. You can take a water sample to the lfs
and they'll test your parameters. Ask for ammonia, nitrite, and nitrate. pH wouldn't hurt, either. That way you know your starting point. Ask for the number results and not just "It's fine" or "it's high". Good luck!