My opinion is that mycobacterium infection is "permanent" ... while it may go "underground" for a time, fish remain infected, and an active disease state is only a stressor away, eg, transport, unsuitable environs (including inappropriate tankmates, low water quality etc), some other predisposing illness/infection etc, etc
The only "cure" is euthanasia & complete tank breakdown & sterilization (not a simple matter as indicated in the artcie)
OTOH some ornamental fish species seem notably more susceptible to mycobacterium ... rainbow fish (I'm undecided about incidence in pseudomugil species, certainly it is much less discussed among these species - initially only wild caught forms were available long after farm bred had become the standard source for the larger rainbow species), Bettas, gouramis are definitely among the IT Fish
I stopped keeping Rainbow fish several years ago
I keep Bettas as I tend to keep them fairly isolated - small tanks, often species tank
I'm a keen fan of Sphaerichthys "chocolate" gourami species & these tend to form the basis of any community tanks (ie any other fish are selected as suitable companions to the "choco's" )
I have lost shoals of choco's to what seems likely to be mycobacterium BUT without necropsy this is ONLY a guess
I've never lost the companion fish in these tanks to similar symptoms - even after complete loss of the choco community - this happens over 2-3 years ... note Sphaerichthys species are not easily kept long term, it may also be a virus or iridovirus (often relatively species specific) rather than a mycobacterium species
I've not observed any significant shrimp losses
I keep my tanks very "clean" re large water changes every 2-3 days (occasionally weekly BUT S vaillanti definitely prefer very clean water ... they are much more active), limited nutrients for plants, fish prefer frozen foods so again less of a complex mixture than prepared foods (especially upon breakdown)
Perhaps with less attention to maintenance, disease transfer to companion fish might be an issue
- the main predisposing factor in fish illness is poor water quality
I also use a Python Water Changer - I always thoroughly rinse the hose etc between tanks (it tends to be well rinsed anyway with tap refill water) & will also do a "hot water rinse & hold" (hot water runs 130 - 135 *F) before the next tank
You could run a bleach "bolus" inbetween but I rarely do this as I'm concerned about chlorine residues re sensitive fish & shrimp
I suspect the "syphon" tube is more a contamination concern than the hose as it remains in the various tanks for several minutes - you might consider investing in separate syphon tubes
Aquarium grade UV sterilizers are very minimally effective (artcile also discusses the UV enegies required to impact various fish pathogens) - if you had one already, carry on
But I'd not invest in one
Rainbowfish colonies have been wiped out even with decent UV sterilizers inline, so it's not any sort of preventive guarantor
To maintain tank "coolness" - keep lights off, add floating frozen water bottles or actual ice blocks, fan on water surface etc
I doubt the recent heat wave was a determining factor factor in your fish illness presentation. ... likely it's just "time"