You need to apply several diagnostic methods, like as other aquatic fish. For example, tilapia alevins can homogenizate as a pool sample (10-15 organism per sample), to larger organisms yo need to dissect each organ to be analyzed (spleen, liver, brain, kidney, heart). There are few images of tilapia, you can get some photos from Dra Gina Conroy, check my paper about Aeromonas too.