Whether or not there is an acceptable pronunciation, there is a proper one. For example, I live in an area now where people pronounce the word "milk" as "MEH-LK" and the word "elementary" as "EH-LEH-MEHN-TARY" rather than the proper pronunciations. While there are regional differences and various dialects, there are proper ways to say and spell words.
For Betta fish, it is always spelled Betta. A capitcal B is correct because of the latin name, Betta splendens, which can also be shortened to B. splendens. As I stated before, "beta" is the Greek letter, which is always pronounced "BAY-TAH" due to its origin. The fish has a different spelling for a reason and is properly pronounced "BEH-TAH," though it does not make a huge difference.
Fun fact: "Plakat" is actually pronounced "PLUH-KOT" officially, though many people say it phonically as "PLAH-KAT" or "PLAH-KIT" as it appears to be pronounced. While it is not completely important, the official way to say it is "PLUH-KOT."