Odd one isn't it.. The fish are from what I would say is a very well maintained and reputable aquarium shop so I am confident they were OK when I got them. I was thinking to setup my old Fluval Chi as a quarantine tank and put the one black fish into it but I am concerned it being alone would only stress it out...
This would just be extremely stressful to the lone fish - new tank surroundings, no mates, changing water parametres re the tank is not cycled (you can do 25% water changes every 12 hours to limit ammonia/nitrite but this is still not as efficient as a cycled filter).
You might check with the shop whether these fish were wild caught or farmed,
sometimes wc can have some odd latent parasites/viruses but this is rare & treatment is generally just supportive care: excellent water conditions, non stressful tankmates, suitable foods, suitable aquarium size/décor re the fish's immune system is best suited to deal with whatever.
Whenever you start with medications, it's recommended to optimize oxygenation of the water, anything that ripples the surface or "splashes" will do this (whether an airstone or pump return), removing/breaking up surface film will help with air exchange, lower water temperature means more dissolved gases ...
- the frequent water changes should improve (reduce) the surface film
- I don't think there's anything you can do with the intank filter to ripple the water surface (or "splash" - gently as most fish will find rigorous "splashing" stressful)
- look at adding the chi filter to help with surface movement (no idea how practical this is as I've never looked at the system)
Something I'd missed in your earlier posts was consideration of removing the sponge or ceramic noodles - definitely don't do this!
both will contain filter bacteria (good) + debris (not so good)
The bacteria associated with debris tend not to be the sort that are efficient N-cycle bacteria, they are however usually fast growing, opportunists so it's "good" to limit these while encouraging/supporting growth of the much slower paced N-cycle bacteria ie gently rinse to remove debris, always use tank water or dechlorinated, similar parameter tap.
If you weren't already treating the tank with unknown chemicals (the eSHa products), I'd suggest gently rinsing the sponge to remove debris - though perhaps you do this already as part of your tank maintenance? - as this would likely help reduce that surface scum BUT at this stage, just use the daily water changes to do this.
Don't rinse the sponge or ceramic noodles for 7-14 days after the last treatment, obviously the 14 day is more conservative ... again you should continue with the frequent 25% water changes during this period following the last application of eSha chemicals.
The "black" sponge may just be that, dark material, other times it's an indication of small amounts of carbon/charcoal integrated into the sponge - after a couple months life in a tank, this amount of charcoal is usually "exhausted" & it will merely act as another surface for your "sticky" N-cycle bacteria, ie I'd not be concerned about removing the sponge at this time in terms of any charcoal it may contain (you mentioned your sponge is well aged) BUT I would be concerned re the N-cycle bacteria population (that would leave along with the sponge).
Once you get the tank back to normal (re no more medications & usual water change regime), then you can sort out a schedule for rinsing the sponge & filter to remove debris - never rinse the sponge & ceramic noodles at the same time. The sponge should be collecting/trapping the majority of the debris so you may want to
gently rinse this weekly or monthly depending.
The ceramic noodles should never need to be rinsed more than a few times a year & always wait a couple weeks after the sponge disturbance.
In a suitably stocked & maintained tank, gentle rinsing of the filter media should not noticeably upset the N-cycle but it does open the door to mistakes, so this is often not suggested by shops/suppliers etc.
I'm going on about all this maintenance to remove debris as I'm assuming the surface scum & lost fish reflect higher levels of dissolved organics in the water column.
If you do a sponge rinse & there is very little "murk", there really is no debris etc concern (& my assumptions are completely invalid).
Lowered water temperature to increase dissolved oxygen: look at the recommended temperature range for your fish before embarking on this path.
Seriously Fish:
embers 20 - 28C
glowlights 24 - 28C
neons 21 - 28C
So if your tank is running at the upper range, you might slowly lower temp to 23 - 24C (adjust heater down 1C every couple of days ... again this is the conservative approach re tank is in a state of some confusion)
(you mentioned that you had more of the glowlights & neons, but have lost fish numbers of both - give some thought to this)
Patience is key when sorting out tank/fish issues, don't jump from one treatment to the next - especially when you have no idea what the chemicals are - even using carbon/charcoal to (
possibly) remove chemicals between treatments, fish may remain stressed by a previous treatment for days to weeks even after the chemical has been removed from the water column.
Another note about carbon/charcoal removal of chemicals, it's usually a process of 12 - 24 hours not a couple hours, & even then, depending on the chemical & other dissolved compounds in your water column, the process may be incomplete (carbon/charcoal "activation" & pore size are contributing factors to this process, most carbon/charcoal sold into the pet trade is low grade/efficiency).
Fortunately fish are very good at managing despite us