If you already own the UV, it can’t hurt but it’s likely worth buying a new UV bulb (depending upon age and use, especially ON/OFF events)Is it better to try and flash the water column with UV and do some heavy water changes?
Unfortunately most aquarium grade UV’s lack the intensity, or intensity + dwell time, to do significant damage to any of the common fish pathogens - though they can be useful for water clarity
Marine sand can be very good at binding medications (eg copper is always a concern but this is only likely to be released at levels that affect shrimp, not fish), “inactive” parasite stages can also go underground as it were in the sand (likely coincidental to sand sifting/shifting movement rather than as a survival strategy ... except it rather looks as if certain strains of Columnaris might be doing just that with respect to natural water sludge environments)
Sooooo I always harbour a certain suspicion of sands ... but would not encourage you to replace yours
You should check for any pH, GH, KH effects
In the last year or so, I have seen shipments of Rummy’s, Cardinals, Neons arrive with what I suspect is some strain(s) of Columnaris (this is a tricky pathogen as the same strain in the same batch of fish, which are then separated to different environments, may display significantly different disease progressions)but the Rummy Nose we’re passing things around for a while and I was losing them until I treated and did a huge water change
Hikari BiFuran is a med of choice (for the home Aquarist, note that a similar, differently branded formulation is nowhere near as effective and much more stressful to fish - but I suspect you can source neither, though it’s always worth checking with your vet (again regulations vary))
http://www.hikariusa.com/water_quality_folder/bifuran.html
It doesn’t “cure” Columnaris but it significantly improves survival rates
Soft acidic water, lots of water changes, lots of water/fish, no substrate (re C’s ability to slowly “tick over” in sludge environs - though I’m doubtful this happens in a well planted, good circulation, frequent water change aquarium substrate), lower temperature 22-23C all act to improve mortality rates for Columnaris infected fish
As mentioned previously, any stressor (for fish), can dramatically reduce survival rates (resistance to infection, ability to develop a rapid and effective immune response against the Horde of potential pathogens that are part of normal Fish Life)
Fish should show no signs of anything but perfect health for at least 2 months, before assuming that Columnaris is absent rather than just being held at bay by fish immune system (change something so fish become stressed and suddenly you’re back to identifiably ill fish)
Maybe this is what’s happening in your tank, maybe not ... without sacrificing fish for necropsy you can’t know (& you still need sufficient numbers for this to reliably represent a group)
It’s equally likely that new fish just came with new friends/old enemies
As for the 40L tank, although a bare tank is optimum for monitoring & controlling pathogens, fish stress level may be equally important
Some fish species/groups appear very relaxed in bare tanks with suitable (dim) lighting (vibrations and any “looming” predators also act as significant stressors) and sufficient numbers, others maintain considerable stress levels regardless (imagine recently wild caught altums)
If tank is long and narrow with sufficient swimming space, it may be worth a go - Neons are usually much more comfortable in smaller glass boxes than Rummy’s
So let ‘em live thereTwo keep going over my guards and into the sump which is a real pain.
In Summary
You can’t go wrong with large frequent water changes
Reduce temperature to lowest manageable, looking at possible Rummy species, I’d choose 22-23C - this should slow everything down and allow you and fish time to “breath” (Neons are more adaptable in this regard)
Vacuum substrate, remove visible debris as much as possible
Keep filter as clean as possible (eg weekly prefilter change)
Feed little, often
If it’s possible to reduce light intensity without triggering plant unhappiness, then do that as well - though happy plants generally mean happy fish ... you might just reduce length of photoperiod instead and come out ahead
If after 3-5 days (with water changes), Rummy’s are looking decidedly unhappy, slowly increase temp to find the lowest they are “happy”
I would maintain the status quo for at least 4 months before bringing in any new fish
Plan to quarantine fish before adding to community
Some people will move a few community tank fish to the Q tank for a couple weeks before finally adding new fish to main tank (but again the Q tank needs to provide a suitable environment)