Sure, so essentially you just click on the object you want to change, then in the panel that shows up on the right, click on "Texture", then where it says Filter, set that to "Nearest" instead of "Inherit"
Object > Texture > Filter > Nearest
If you have the object (fish in your case) saved as a scene I believe you will need to right click it, then press "Open in Editor" and do the previously mentioned steps in that menu.