Here are some reasons why:
* Bettas need at least 5 gallons of water. This is because they are active fish that need room to swim and explore. A 1.5 gallon tank is simply too small for a betta to thrive.
* Neon tetras are schooling fish. This means that they need to be kept in a group of at least 6 individuals. Two neon tetras will not be happy in a 1.5 gallon tank, and they may become stressed and aggressive.
* A 1.5 gallon tank is not able to provide a stable environment for fish. The water temperature and pH can fluctuate too easily, which can stress the fish and make them more susceptible to disease.
If you are interested in keeping bettas and neon tetras, you will need to purchase a larger tank. A 5 gallon tank is the minimum size for a single betta, and a 10 gallon tank is the minimum size for a group of 6 neon tetras.