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...