First some information like how big is the tank in size (water capacity) and what type of water conditioner are you using to remove the
chlorine or
chloramine from the water? Is the tank filtered? Is it heated? What is the temperature? (should be at 78 to 80 degrees for a betta) How long has the tank been up and running? How many times do you change the water and how much at a time? (if the tank is not through the
nitrogen cycle the answer to this should be daily and half of the water treated with a dechlorinating agent like Prime, StressCoat, NovAqua+, Amquel+, or Amquel) The Nitrogen Cycle can be read about at:
http://www.fishlore.com/NitrogenCycle.htm
Second, the shrimp can or cannot be breakfast, lunch or dinner depending on the betta and there is really no way of knowing as every betta is different.
BUT shrimp is their first food and during the molt of the shrimp's shell he is very vulnerable to the betta. Nothing is impossible. And you cannot be mad at the betta for doing what comes naturally. They are a natural food source. Then, crabs NO they would make a lunch of the betta and the shrimp. Sorry but the tank if small enough may do for the crab and get a larger tank for the betta. The shrimp I do not know. He would probably stand a better chance with the crab.
It would be my suggestion to read the Welcome and Betta Card Guide at the beginning of the posts of the Topics List of the Betta Board. I do not believe that the friend who gave you the tank and betta gave you much information and it does give you most of the beginners information about how to get started with betta care and what the generalities are that they need and will have to have to stay healthy and alive. They are Tropical Fish and as such demand much more care than pet store employees and owners and the people hawking equipment out there will tell you.
Welcome to Fishlore.com and I do hope that some of what I have said here will help you and your betta. You have come to a place with a wealth of information about fish and bettas. We have many members who can help you and give you advice from their experience with their fish and will be glad to help.
Rose
http://www.fishlore.com/fishforum/be...ead-first.html