Unfortunately my betta died the other day. He developed a discolored area on his tail that spread to his body and looked like mold (hairy like). The pet store was closed and the next day he was dead and covered in a white hairy looking fungus. He was in a 5 gallon w/ 3 other tetras. Should I worry about the other fish getting this fungus or is it just a random thing? All of the other fish look fine, just want to make sure.