I can't say,the growth rates are different depending on the species.
From what I know,clean water + sufficient amount of food promotes growth too.
Some spurt like crazy but some just grow 1'+ or so for half a year.
As for the food,I would recommend hikari's cause the amount of vitamins and stuff are balanced throughly.
Try massivore delite or sinking canivore pellets.Fed mine with hikari's sinking goldfish pellets.
Earthworms are also bichirs favourite among all,never did heard before any bichir resisting earthworms.
For treats,bloodworms,nightcrawlers,earthworms as I mentioned,maybe shrimps,etc.
The best is keep it a varied diet,this would bring out the best in them.
