Did you tell your LFS you was trying to keep other fish with a betta in a 5g aquarium? If you did and they didn't tell you it's a recipe for disaster then I would consider taking your custom elsewhere as they don't know the basics.
A 5g tank isn't big enough for either tetras, corys, or algae eaters. Mix in the fact that bettas are extremely territorial, and there simply isn't enough space for these fish to co-exist. Unless you can come up with symptoms that would suggest another cause, it's a fairly good bet your betta just killed the other fish. And if your bettas didn't kill the other fish, he would have probably got around to it eventually.
It is possible to have fish cohabiting with bettas, but there are measures you need to take.
- Make sure there is sufficient space for all the individual fish to find their own space and establish their own territory. 60 litres/ 15 gallons would be the minimum I would consider trying to keep other fish with bettas. Consider what space the other fish needs to. For example depending on what species of algae eater you got, that might need upwards from 100 gallons. Some tetras and corys need 20 gallons or more.
- Select the other fish species carefully. Some have a better chance of cohabiting than others. Avoid anything that is too showy with flowing fins, like guppies, that a betta might mistake for another betta. Avoid fin nipping fish that might target a bettas flowing fins. Neon tetras and corys are safe picks, but nothing is guaranteed.
- Add the betta last. If the bettas has been living on his own he will have decided the whole aquarium is his territory and will be more likely to react with extreme prejudice to newcomers.
Consider that whenever you mix new fish there is a risk, but more so with bettas. It will come down to the temperament of individual fish. One betta with a mild temperament might accept tank mates, while another more territorial in nature simply won't. One group of tetras might be peaceful and leave your betta alone, while a different group of identical fish might fin nip the betta to death. Always have a plan in hand to separate fish in case they don't get along.
Finally, bettas really don't want company and are perfectly happy on their own. Adding tank mates is for your benefit, not the bettas.
