Sexing fish from a tissue sample
As already pointed out by Remi, you can identify the sex from a tissue sample only if there is a genetic (such as chromosomal) sex determination system (by using techniques such as PCR or in-situ hybridization). However, different species of fish employ different mechanisms of sex determination:


  Focus on or attachment to genetic systems involving X and Y
  chromosomes must quickly be shed in studies of sex determination in
  fish due to the diversity of mechanisms utilized, and, by doing so,
  the researcher can explore alternative regulatory mechanisms that hold
  important clues for understanding sex determination in general. The
  biology and ecology of fish is sufficiently diverse to provide unique
  examples of sex-determination mechanisms, yet they possess many of the
  same processes and pathways that are used in other vertebrate systems.
  
  
   Devlin, Robert H., and Yoshitaka Nagahama. "Sex determination and sex differentiation in fish: an overview of genetic, physiological, and environmental influences." Aquaculture 208.3 (2002): 191-364. 


Though Devlin and Nagahama comment (above) that fishes also employ the same signalling pathways as other vertebrates for sex determination, study of these pathways to determine the sex would be extremely difficult as you may be aware that the cell-signalling and the gene regulatory networks are highly complex.

Fishes are highly sexually dimorphic (including Discus) and the sex can be easily identified (with some practice) by visual inspection. Microscopic identification would be both money and time consuming. Morphology based identification can be automated (when the machine is trained sufficiently). You can have a look at this article1 which is about using image analysis to score salmons. 



1.&nbsp;&nbsp;&nbsp; Merz, Joseph E., and William R. Merz. "Morphological features used to identify Chinook salmon sex during fish passage." The Southwestern Naturalist 49.2 (2004): 197-202.