Neon Tetras with white 'patches' and 'torn' fins

I've had my neon tetras for a few months and noticed recently that the two females had 'torn' or 'eaten' fins and tails, as well as some white patches on their sides which looked to me like lost scales. I just assumed that one of the others had been nibbling on them, but I've never seen that happen.

Today I noticed that one of the males suddenly has the same fins and patches as the girls. I tried to look up the symptoms listed on here in an attempt to figure out what was wrong but what I found was too vague for me to tell whether it applied to my guys or not.

Also, I recently needed to give my axolotl regular salt baths to cure a skin condition he developed due to the high temperatures this summer and I was told it works for all fish. Would it be worth giving this a try and, if so, how do I go about it.

I've posted some pictures on here for you. The fins are easy to spot but the patches were really hard to photograph. Try looking at the sides of the girls (the only 'fatties' in the bunch) as you can somewhat see in some pics what I'm describing.


Please help as I love my fishies and don't want to lose them if I can help it!!

Water quality: (as of a week ago before i changed out the whole tank of water in order to add some newbies)
*Ammonia: 0.25ppm
*Nitrite: 0ppm
Nitrate: 160ppm
*pH: 7.2
*Temperature: Between 24 - 28 degrees Celsius

Tank details: 28L tank with heater, filter etc, no live plants as yet
Tank Inhabitants (numbers, sizes ect): 10 neon tetras (2F, 8M) and a bristlenosed catfish
How long the tank has been set up: 5 months
Frequency and amount of routine water changes: 1/3 change weekly, full change to a newly cycled clean tank once a month to clean usual tank
What you feed , how much and how often: small pinch of Aqua One micro for tetras, three times daily
Water conditioner used: Showmaster Water Conditioner (blue crystals), Seachem Prime and Seachem Stability
New fish or plants added to the tank? New fish added nearly 2 weeks ago but the problem existed prior to their introduction
Were they quarantined: They were kept separate for a couple of days
Medications used: None as yet
Describe the problems you are seeing: fins which looked 'eaten' and sides of fish which appear as though scales are missing/covered by something whitish

Filtration details:
Type of filter (external, internal, under gravel): external filter
Maintenance details: cleaned out / rinsed twice per month, filter media changed monthly
Media used: foam pieces and charcoal (black pebbles?)