Yesterday I got 6 neon tetras and a male betta. They live in a 10 gallon tank that's 79 degrees Fahrenheit. Today I woke up to find one tetra in the filter, and 3 dead. As of right now I have one healthy tetra, and one who is swimming weakly (upside down, sideways, etc.). The betta doesn't seem to be bothering the tetras, so I don't think he's the reason they're dying. What could be causing them all to die?