I need some help with making a texture to my mod

Ok, so I'm making an aquarium to my mod where you can insert an item to make it look so there is the fish you put into the aquarium on the inside. The script is pretty much done, but I can't get the fish on the model. The fish texture I want to use is the raw fish texture. It's a Minecraft texture, so I can't get it as a layer on the aquarium texture. Hope you can help me with this.🙂