Three tools I use that aren't fish tools but make my hobby much better.
1. The thin, white spatula: This came with my immersion blender and I love to use it to scrape the sides of my micro and banana worm cultures when I don't feel like sticking my finger in there.
2. Substrate scoopers: These are pet supplies but they're not for marketed for fish. The small blue one is sold for hermit crabs. The other two, I believe both found at Petsmart, are in the reptile area. GREAT for scooping duckweed from the surface. Also great for scooping gravel-sized substrate and rinsing it. The two larger ones are great for getting into corners...if you're tearing down a tank and you can tilt it-- throw in some water, stir it up and you can scoop out the last gravel more easily from the corner(s).
When I put my snail feeder-catcher down or some repashy and I want to pull a bunch of snails, these scoopers are handy to grab the ones nearby.
3. Red paint scraper from the hardware store: When new, great for scraping algae (the edge can dull over time depending on how you use it,) if you've let it go for a while and have a sheet of it on your tank wall. (This kept happening to the one tank whose side faced a SE - facing window until I painted.) I've also used this to pull java and other mosses with hair algae to provide a surface to separate (with a bamboo skewer) what I could. Also used this to pull duckweed from the top but it's not as good as the substrate scoopers.
I'm sure there are others, so I'll add to this when I come across more. Do you have any non-fish tools that are amazing for aquarium work you can share?