Apologies for delayed reply (and in advance for my brevity / struggling due to ill-health).
Difficulty seeing photo - any further ones would be helpful.
Are the bumps white like breeding tubercles (commonly found on pectoral fins)? (Do an internet search for what these look like.) Ich is like grains of salt and all over the body, so unlikely, esp if hasn't spread.
Nothing else sounds untoward, so wouldn't medicate. More frequent and larger water changes, or sometimes the occasional dose of aquarium salt, can resolve a problem of unknown origin.

My immediate thought, on seeing the photo, is that it's one of these https://en.wikipedia.org/wiki/Physidae which often arrive via live plants. If so, eggs may be being laid, and numbers thereby increasing. To stop numbers increasing, check for signs of eggs on the glass, around the tank, and especially behind the filter, and remove any that you see.

It is possible that pieces of filter cartridge (eg carbon) could dislodge which is a less likely explanation than the above; if it is this, you might need to consider transitioning to an alternative filter media or putting the foot of a pair of nylon tights over the internal filter output to prevent it disintegrating and ending up in the tank.

Test strips are far from accurate. Once we know your actual results from liquid-based tests, we'll be able to advise further as those results will help us determine the appropriate course of action. [No need to add a photo if you can't - just type/dictate them into the post.]

Jeyes Kleen-Off Ammonia = good.

24 hrs.

Can only reply v.briefly but, in essence:
* nice tank - nothing looks untoward;
* what are ammonia/nitrite/nitrates readings? - something even slightly awry might a/c for behaviour change;
* v.possibly, they've been spooked by something external/internal to tank - can only recommend softly/delicately moving towards tank (if poss), sitting by it for however long it takes for them to get used to you, & even chatting/communicating w/ them (this calms mine down and/or they are bemused by it & come out to investigate).

What are the precise readings for ammonia/ammonium, nitrite and nitrates?
Have you used anything in the same room as the tank that could be potentially toxic (eg paint, air fresheners or other aerosols)?
I definitely wouldn't say it's the end of the road for him yet. A few points:
* if you have a spare quarantine tank, heater and filter (into which you could put a portion of the mature filter media from the main tank filter), it might be best to move him into that and see how he fares out of presence of the othersl
* also worth checking to see if he still swims in the same way with the filter switched off for ~15secs, or whether he reverts to normal swimming - it might be that he needs a gentler flow for a while until the issue resolves itself;
* does he have a damaged fin (eg pectoral fin)? That can sometimes affect swimming and incorrectly make it seem like a buoyancy/swimbladder malfunction.

Of the 5 species I've had during the nerite snails' time (various tetras and rasboras), only the cardinal tetras showed a very brief interest in pecking any - which soon waned (and it was only those on the glass that intrigued them).

They're definitely nerite snail eggs.
They won't develop into nerite snails in a freshwater tank. My bogwood is absolutely covered in it over the 3 years that I've had a female nerite snail inhabiting the tank. :)

When you say 'cycled', do you mean you completed a fishless cycle on the tank by adding an ammonia source (a 6-8 week process usually to ensure it's cycled) or adding mature media before transfer?

If so, then it's possible that he's seeing and playing with his own reflection, or learning/exploring the expanded boundaries of his new tank, or possibly getting used to / stressed by the more active danios zooming around now that they have more space. He'd benefit from more company of his own kind, and that would help him settle, but I appreciate that he may be the last of his shoal and you may not want to get any more.