I dont know enough about your tank to make a guess.
Illnesses fall in 3 categories:
Environmental stress: declining water quality, stress from improper conditions like temperature and hardness, and stress from incompatibility between species. Environmental issues tend to be slow and cumulative. They may not kill your fish outright, but they will allow illnesses to get a foothold in time.
Have you done any water testing today: ammonia, nitrite, nitrate?
What fish in tank and how many?
Nutritional deficiencies - What do you feed and how often?
Most issues begin with the first 2 and lead to the 3rd.
When was the last time added other fish to aquarium?
This erosion of fins- that just popped up in one day? Thats unusual.
It isnt unusual to get shedding of slime-coat in one day to some kind of irritant- whether it be a toxin that got into the aquarium, ammonia/nitrites, or water added to aquarium with lots of gas bubbles. Are the fish darting around as if trying to get away from something irritating them?
If thats the case it could also be a parasite: such as Velvet.
Or- it could be a bacterial infection. A particularly aggressive one.
Are the fish still eating?
Need more information.
1.I just did a 50% water change on Saturday night. However, doing a water test today shows
PH: 6.0 (or below. Yikes!)
2. I've been feeding a variety of flakes, frozen, and baby brine shrimp.
3.It's been a few months since any few fish was brought in. The fish were very actively eating the baby brine shrimp today, but are all listlessly hanging around on top of the surface.
I think it's clear i need to do an emergency water change to try and fix the tank params. I think I also need to test the tap water and see if there's anything up with that. Ph has always been around 7 in the past.