Neon Tetra Disease (I Think)

After a wearing 8 months (Dec2019-July2020) with 9 fatalities - the last 3 of the elderly x-rays, 3 elderly harlies, and 3 young cardinals, I really hoped it would be a fresh start with the new additions added last week following 4 weeks' quarantine. Not to be, it seems!

One new cardinal started displaying all the characteristics that seemed common in the 3 cardinals who died - rapid breathing / faster gill movement than the others, dark spine turning white the full length of it, body very pale (especially as soon as lights out and for a long time in the morning), bloating (visible when viewed from above), gill losing colour. He's too small to determine whether he's got the 'fuzziness'/borderline distended scales when viewed from above but has additionally developed what initially looked like a lump/cyst/spot similar to the second cardinal that died but on the edge of his gill - and, additionally, it had white fungus sprouting from it. To my horror, Wee Harley, my last remaining harlequin rasbora of whom I'm very fond and protective, actually tried snapping this off the cardinal's gill yesterday morning - but without success. This is a lot worse today with a visible white marking - photo attached - but, ironically, the whitened spine and pale body has 'improved' / is scarcely visible today although the bloating remains. One of the older cardinals has also been breathing faster, although not as fast as this cardinal, but so far there are no other symptoms in that particular fish (touchwood).

I and Mr FCMF tried in vain yesterday and today to catch the affected cardinal to remove him for treatment, including during yesterday evening's water change when the water level was low and all decor removed, but to no avail - those cardinals swarm in protectively round one another and are extremely good escape artists! I wouldn't be in the least surprised if this is going to spread to the others given their close proximity and WH's attempt to remove the spot and then tight shoaling with her new shoalmates...

As a recap, 1st fish responded poorly to Epsom salt baths, isolation/eSHa2000 (Dec2019); 2nd fish died comparatively swiftly within days while deliberating whether/how to treat (May2020); 3rd fish delayed treatment due to repeated difficulty capturing although used Melafix in main tank, but died just after deterioration and capture/isolation/eSHa2000 (July2020). I've resorted to Melafix-ing the main tank for the past couple of days, if only for the sake of feeling I need to be doing something but not keen on subjecting the entire tank to treatment. Aware that Waterlife Protozin has some evidence of potential effectiveness, I enquired elsewhere about its ingredients, and in the process received some extremely helpful advice about possibilities which I'm currently exploring while trying to figure out the best course of action.

There's definitely something to be said about a single-fish tank - no need to consider other inhabitants, invertebrates, removal for treatment, etc.! Whatever way this evolves, I won't be getting cardinals again, that's for sure...

Sounds like a potential step in the right direction @fcmf
Keep us informed! 

Thanks, both.

I did try to find the fungus and white 'thing' as I was keen to remove it - and would be delighted to have it inspected if I did/do find it. I'm conscious I do a lot of tinkering in the tank, and didn't want to have to take all the decor out yet again in an effort to find it when the fish are already in and out of "terror mode" - caused by the net a few times in an effort to catch the afflicted fish, and the Melafix (which had never been a problem when treating the last afflicted cardinal in the spring / earlier summer). There was something white floating off a leaf a day or two later but I wasn't convinced it wasn't snail poo. I'll have a concerted look once the tank is emptied of decor for the weekly water change tomorrow.

Although the fungus/'thing' aspect has disappeared, what I have gathered from this almost-certain NTD is that the fish is never going to recover as such - but it's a matter of trying to keep it as comfortable/happy/stress-free as possible in what limited life it has. The first was very stressed by and didn't respond well to treatment at all, the second had a relatively swift demise, the third had a relatively lengthy but seemingly stress-free demise (6 weeks). Having tried the "Mr FCMF test" (ie can he see which fish is sick?) yesterday evening, the fish was easily identifiable once again as its spine was very white and red stripe very pale and its breathing today is extremely rapid - but this may wax and wane again. If I think I have any chance of getting it captured this evening, though, then I might bring the water change forward in order to give it a chance at capture and isolated treatment - but not if it's clear the fish is going to be more stressed by the experience than if left in the tank with its tankmates.

The other aspect is that the NTD parasite may well be in the tank, and there are options for attempting to get rid of it. One would likely mean having to set up another permanent tank for the nerites (argh - no space!), and the other mightn't mean that - but both would mean subjecting the fish to extensive courses of treatment (which doesn't exactly enamour me - but I might be tempted if I find others coming down with symptoms). Food for thought...

Keeping fingers and fins crossed 

Thanks, both. 
Pleased to report that, following my sleepless night, the fish is still alive.  He'd dive-bombed behind the piece of Tufa rock not long after moving him into the hospital tank, hence my fears about him ending up behind filter/decor overnight; however, having removed the Tufa rock, thankfully he didn't end up behind anything else.  The other symptom I'd noticed yesterday was the lump at throat level like Cardinal 2 had.  Anyway, although he looks no better and no worse, he's at least tolerating the hospital tank and medication better than Cardinal 1 who seemed to deteriorate in it.  I had a good 'chat' with him through the glass and he interacted back, so I think/hope he understands that I'm doing my best for him and that's the reason he's in there.  The others in the main tank have spotted him and been up at the front of the glass watching him but he's largely been facing in the opposite direction.

As an update:
* after several nights of no or very little sleep for worrying about the fish, I gradually diluted the medicated hospital tank over a couple of days and returned the fish to the main tank - this improved his demeanour from having been motionless on the base of the hospital tank to swimming around inquisitively in the main tank and generally behaving normally with the others including showing interest in food (although I don't actually think he's been eating);
* he's still white along the spine, looks a bit faded in places, is breathing fast, has the lump at throat level, and sometimes looks deathly pale in the morning and at night but sometimes doesn't;
* this weekend's water change let me examine him in detail - having shone a torchlight on him from above, it's clear he's bloated, his eyes are protruding, and, on careful inspection from above, that the faded look is due to the scales sitting out / ruffled;
* option is either re-hospitalise and treat with eSHa 2000 (but at expense of his demeanour/wellbeing and risking an unhappy death on his own, and may just be postponing the inevitable) .v. keep him in main tank for his demeanour/wellbeing (but negating any opportunity for improvement of which I think there's little chance even with medication) - for now, I've gone with the latter but with the addition of Melafix to the main tank as a 'halfway house'... it's a situation of ongoing monitoring daily, though, and potential change of plan at any stage.

How are things this morning?

I didn't anticipate this - the fish is more alive today than yesterday... more transparent than milky-white coloured. Perhaps the inevitable isn't going to be just as swift as I'd anticipated (famous last words). S/he seems content, and no others are showing any ill-effects (yet), so I can't hope for more than this.

I can't be certain that this, and what the previous 3 cardinals suffered from, is definitely NTD but it's the closest match to the symptoms [source: https://www.thesprucepets.com/neon-tetra-disease-1378484]:
Restlessness - NO
Fish begins to lose coloration, often in one part of the body - YES, DEFINITELY [MAIN SYMPTOM]
As cysts in the muscles develop, the body may become lumpy - YES, IN THE CASE OF AFFECTED CARDINALS ?1, 2 & 4 (AT THROAT)
Fish has difficulty swimming - NO
In advanced cases, the spine may become curved - NO, THANKFULLY
Secondary infections such as fin rot and bloating - YES, BLOATING

Other possibilities might be some form of bacterial infection or velvet disease. I know very little about the latter but the gold/peppery colour often described is not being experienced, unless this manifests itself as a white/transparent spine in a cardinal tetra.

Thanks, LF.

The latest is that the fish is still very content with its tankmates but breathing even more rapidly, the white areas are looking a lot more 'ruffled' and there's a bubble on the enlargening lump at the throat area. Cardinal 1 developed this shortly before death - I can't get a good photo but a bit of googling suggests its dermosporidium/dermocystidium which seems in this case to be secondary to ?NTD.

Sorry to hear that the fish has finally passed.