Introducing New Fish -Rainbow / Dwarf Shrimp: Are they toxic to other fish species?

[Sepceli]

My parents recently acquired some new fish for the aquarium. We have had a balanced tank for a long time now with no real big problems. Our tank has been filled with a variety of Tetras from Neon and Black Tetras to some Phantoms and some bottom feeders, we even had a male Beta and they were all getting along great. We did introduce some Red-Eyed Tetras to the tank but we quickly returned of them as they started to become aggressive with the Beta and chew on his tale. A few days after we removed those Red-Eyed Tetra and introduced the new tetra to the tank, Green Fire Tetra and some others that were more suited for pond water. Our Beta didn't like them, though they didn't have a tail to make them angry they were not a Tetra either. So we moved them to the pond. Anyway we also got two shrimp as well. One of the shrimp has disappeared and could be molting but shortly after that shrimp vanished our Beta started hanging out at the bottom of the aquarium and then died last night. It was heart breaking as he was the star of the show.

Anyway, I wanted to know if perhaps the introduction of some new fish could have made the water toxic to the beta? The water temperature is fine so that is not an issue. Or if it is possible that the shrimp that was going to be molting could have become a victim to the Beta and made him sick. I know they are bottom feeders and I was wondering if they could be toxic to other fish if eaten. We still have one visible shrimp left and I wanted to know if we should remove him, due to the fact that I notice some of our Phantoms and larger Tetra take an interest in him when he is swimming through the water and not blending in with the ground or our drift wood.

Thank you for taking the time to read this. Please any links or helpful information would be great. I have looked up some of this information but I am still missing some keys to the puzzle, specifically those listed above.

Thank you for your time.