Personally, any Bichir under 15 inch I’d put in a 40 breeder but as a bare minimum obv bigger the better. Since 3 Ft by 18 inch should be sufficient for a fish that size (main concern is the width). I’ve kept my 15 inch platinum endlicheri in a 40 breeder and it’s finally getting to the point where it’s getting too big for it. Any upper jaw Bichir outside of the teugelsi, ornate, and weeksii, imho can be kept in a 40 breeder if the stocking is low and maintenance is kept up. If you can get a bigger tank go for it but the dimensions are more important than the gallonage. 75 gallon (4 Ft by 18 inch) for 1 upper jaw Bichir or a small group would be ideal and 150 g would be best for a larger group (5 ft by 24 inch).