The rapid coagulation of fish blood makes the use of anticoagulants in hematological analysis necessary and means that choosing the most appropriate product is important. The aim of the present study was to evaluate the effect of two different anticoagulants (heparin sodium and K2EDTA), storage time, and temperature on the hematological values of Arapaima gigas juveniles. Twelve juveniles (905.79 ± 111.2 g and 49 ± 1.74 cm) kept in captivity were used for the tests. Blood was collected from the caudal vein and divided for the analysis of the effects of K2EDTA 10% and heparin sodium 5000 IU, storage time (0, 5, 10, and 24 h), and temperature (4 and 28 °C). The hematocrit, erythrocyte count, hemoglobin, mean corpuscular volume (MCV), mean corpuscular hemoglobin concentration (MCHC), mean corpuscular hemoglobin (MCH), and differential leukocyte count were evaluated. The results revealed significant differences in the hematocrit, lymphocytes, and neutrophils when the anticoagulants were compared at 0 h of storage. In the hematocrit, heparin sodium produced variations, while K2EDTA maintained the values constant at 5 h, the erythrocyte count constant for up to 24 h, with the exception of at 4 °C, while hemoglobin produced variations in all the groups at 5 h. The blood indices MCV, MCHC, and MCH were best preserved, without variations, using K2EDTA at 28 °C for up to 5 h. In conclusion, K2EDTA 10% at 28 °C maintained constant hematological values of Arapaima gigas for 5 h, with the exception of hemoglobin, which should be analyzed as soon as possible.