bichir growth rate

So I bought two juvenile Poly Delhezi that would grow in my 200L/52gal tank, to then be moved to a 540L/142gal tank when they have grown a bit. So everything was goin' fine and dandy, they were eating awesomely and they had nice temperaments. Until I noticed one of them was growing a slightly...

Hey all, I have had my female wild caught polypterus lapradei Nigerian for 3 years now (if you check my past posts may remember i asked tons of q about them when I first got mine). She was 11.5 inches approximately back then and now after 3 years is 12.5-13 in. I’m wondering if this is normal...