Only if you want the shark to grow up and eat the betta. A full grown red tail shark gets about 6 - 8 inches long, and they can be quite aggressive and are very territorail. Not a healthy mix for a betta who doesn't swim well due to long and fancy fins that are not designed for good swimming techniques.
Also, not a good idea to put a betta into such a large tank. A 50 gallon tank is quite deep, which will make it difficult for a betta to get to the surface for air. Bettas are air breathing, sharks are not.
The 50 gallon would be suitable for the shark, a tank of 10 gallons or less would be most suitable to a betta. In 10 gallons you could keep the betta with a cory catfish, dwarf frog, or other peaceful fish like an otocinclus.
thanks for your reply the tank is 18 inches high been transfered from a 3 ft x 1ft x 1ft the colour is changing to a light blue from deep red is this a lack of oxygen or because he is happy,his fins are nice and loose floating behind him seems happy
There are a number of things that can cause color changes in a fish. Poor water quality, stress, foods, temp/temp changes, and maturity.
If I knew more info about your tank (water params for ammonia, nitrite, nitrate, pH, temp, foods, and how many decorations are there, etc) then I can help you to sort it out. Pictures of the tank and fish would also help.