The "spider" pic is really much too fuzzy to be able to tell what it is. Don't forget that terrestrial insects do end up in aquariums.
Are you shooting at your highest magnification? Try using a lower power. The higher you go the less depth of field & resolution you'll have. It's better to get a sharper low magnification image & enlarge it with photoshop than to have a fuzzy high mag image.
Thanks, I am going to write to the Professor of Marine Arachnids at the Australian Museum. I don't think my rock is from Australia, but that might interest him more!