Most labs use the values for the normal control fingerlins or animals as references in a study, the other alternative is to measure it against the kits reference, if the kit is standardized for fish. This article may help
Effect of dietary supplementation of chromium on growth and biochemical
parameters of Labeo rohita (Hamilton) fingerlings
ABHAY KUMAR GIRI, NAROTTAM PRASAD SAHU, NEELAM SAHARAN AND
GYANARANJAN DASH 73 Indian J. Fish., 61(2) : 73-81, 2014