although we do heat the betta water at the store i work at, i have several at home that are not heated and they do fine. i doubt its a water quality problem thing because even some of my more fish-educated customers have had this happen and i know they know how to take care of them. bettas dont really mind a small space because they have a lybrinth organ that allows them to use air to get oxygen and in the wild can live for several days laying on top of mud fliping over periodically to stay wet if they are in the shade. also one of my bettas (a dragon scale named shine) has lived in a small cup for about five years.
They can live in small spaces, yes, but some people take that to mean they should permanently live in tiny bowls and vases. As for the temperature, keep in mind that your room temperature might be heating them enough while other people may have colder houses.
I won't comment too much on the cup thing since I'll end up ranting, lol. While they can
live in cramped conditions like that (and often for many years), it doesn't mean they should IMO. I'll never understand why people insist on keeping bettas in tiny tanks--but gourami species (who have the same organ) in actual aquariums.
Your best bet would be to get people to bring in water samples to rule out water quality as an issue. Bettas are really hardy fish so something
must be off. Maybe diet is another factor to look into? What are most customers feeding?
The only other time I usually see symptoms like you describe is when a fish isn't acclimated properly (just dumped in the tank)...is that a possibility? That's not usually as much of an issue with bettas but it's worth looking into!
Good luck, I hope you figure it out
. And again, the main reason I bring up water quality is because most people are very lax with bettas due to their hardiness. When I was really into bettas (had them all over the place, haha), I kept them all in heated, cycled, planted aquariums (I still have a couple as pets, each in a 5-gallon). I have to say, I never experienced ANY disease or sudden deaths. They all lived several years...including the "fancy" varieties. I do, however, have quite a bit of experience treating bettas for disease--all other peoples' fish! It made me wonder if maybe a lot of people are inadvertently making their bettas more prone to sickness