sorry if its having a floating problem, theres really nothing you could do. hopefully its just gas, and it'll go away on its own. ime, i haven't found anything i could do that would positively help. people say to try epsom salt, but it hasn't had an effect for me. basically wait and hope for the best.
edit: i've also tried adding aquarium salt and antibiotics, neither has worked. its hard to say exactly what the cause is, when the bichir develops this problem suddenly. it could be internal infection (maybe?), or it could be trapped gas, in which case, if you wanna be doing something, you could try feeling its belly and see if you feel anything and try to help push the gas out. alas, its harder than it sounds, and it hasn't worked for me yet either. i honestly don't know if it does anything, but desperate times call for desperate measures lol basically, a lot will depend on what the cause is and if it can right itself. good luck!