I'm sorry for your loss, it's never easy losing a beloved animal.
Originally Posted by CWolf
Also I like an over-stocked fish tank as much as you guys, which is to say not at all. All the fish, except loaches and betta, are under 2cm in length so it's really not that crowded.
As others have highlighted, your aquarium is rather overstocked. Whether 15 litres or 30 litres (4ish gallons or 8ish gallons) the majority of your fish would be happier and healthier in larger accomodation. The number of animals an aquarium can handle isn't so much in the length of each individual occupant but rather the amount of waste they produce and their activity level, schools also have to be considered if the fish species is one that likes to live in a group -- not to mention feeding habits.
In an aquarium of 30 litres I would say your other betta and the amano shrimp could live happily without any issues (when provided with regular maintenance) but the platys, zebras, glass fish (glass catfish by chance?) and loaches could all do with being in larger homes. I know that this will not account for the mystery of your worm (though I'm glad to see it's been identified) it may have contributed to your betta's passing and may also contribute to future deaths if something isn't done soon. Large numbers of fish in a small body of water will cause vast amounts of ammonia to be produced (again, this varies depending on species present) and you may find as time goes by your nitrifying bacteria simply can't keep up. You can only have so much bacteria in one tank and it may end up just not being enough.
All fish are sensitive to changes in their water chemistry, some more than others. Glass catfish are a prime example of this and can die very quickly if something goes wrong -- luckily you'll be able to tell if something is starting to go wrong as similarily to ghost shrimp your glass fish will turn less transparent and become a white colour, I just don't want it to get to that stage for both your sake and the fishes'.
If you are at all able to rehome or upgrade the other fish into other aquariums (I imagine they could all live peacefully if kept in larger quarters with an appropriate filtration system, fully cycled) that would be superb but I understand that sometimes we're in a fix and struggle to house things bought for whatever reason (whether it be lack of knowledge on fish store staff's part, impulse buying or a myriad of other reasons). If you struggle, you may be able to get away with temporarily keeping the platys with the betta and shrimp -- not something I'm usually recommend due to the platys' activity levels and bright colours but if you can compromise and monitor your fish carefully you should be able to avoid any more deaths.
May I ask how long these animals have lived together? I know that sometimes fish can get along wonderfully (or seemingly wonderfully) for a while then suddenly snap and things start going wrong, I also know that stressed, unhappy or unhealthy fish can coexist simply because they are too stressed, unhappy or unhealthy to do anything about it.
I wish you the best of luck and hope all goes well in future.