There have been some fairly recent developments in sexing Arapaima with a simple blood draw. To quote an article I found by searching for "sexing arapaima":
"The method involves assaying the blood of the fish for levels of plasma vitellogenin, 17-estradiol (E2) and 11-ketotestosterone (11KT), with the authors verifying the sex of the subjects by sacrificing them and examining the gonads.
From previous studies with other fish species, the level of plasma vitellogenin is higher in female fish than in males, and the 11KT:E2 ratio is higher in male fish than in females.
Based on the method developed by the authors, they were able to sex the fish with 95% accuracy using plasma vitellogenin and with a 100% accuracy using the 11KT:E2 ratio."
Perhaps you have access to a local university that can assist with the blood analysis.
