Do you have a goldfish in with tropical fish? Please say you don't...
Keep in mind goldfish need a minimum of 20/30 gallons for one, as they grow to about a foot long. If the tank is too small, their body will stop growing, but the organs will continue to grow until they end up compacting and killing the fish.. goldfish have a life span of 20+ years. They also don't do well in temps over 65F, and the platies/betta are tropical fish and require temps above 76F - that is a big difference in temps in their world. Goldfish also produce a TON of waste and generally is recommended to double the filtration in their tanks along with a lot of big water changes. Having a goldfish with tropical fish such as bettas and platies will ensure that one or more fish will die quite a bit sooner than their normal life span.
So please tell me your large goldfish tank is next to the tropical tank :)