a gravel vac is a must; the small non-battery ones (a plastic pipe attached to a syphon) work well and are cheap
buckets kept for fishkeeping only
a store of dechlorinator so you can always do an emergency water change
also a couple of unexpected ones:
a toothbrush is really good for those awkward corners where a bigger scraper might upset your plants
a plastic measuring jug IME is better for catching fish with than a net: they get less stressed out as they don't have to come out of the water; also good for mixing up medicine and plant food in, and for adding water back to the tank during water change
for fry I use a plastic soup ladle
a plastic bin bag to put over floor/furniture during water changes
hi
i always have meds for these diseases in our house:ich,fungus & finrot,internal bacteria. and melafix that i find useful for mild fungus/finrot etc. you'll always spot a disease at the most inopportune time (late night/public holiday etc) & its good to have treatment on hand to start right away. i also always have pure clove oil on hand for humane euthanasia an unpleasant side to fishkeeping but occasionally necessary.
A turkey baster (used just for your fish tanks) is a good tool. In a heavily planted tank you can use it to blast out any built up gunk between the plants, in reef tanks it can be used to blast between the rocks, use it to target feed fry some infusoria or baby brine, use it to target feed inverts in a reef tank, or target feed fish that are slower feeders and can't compete against other faster fish. It can also be used to carefully suck up small fry if needed (extra care is needed to do this) or the waste that anemones release.
Brine shrimp nets are good to have too. You can use it for (of course) netting brine shrimp, but it also works well to net fish with barbels, or delicate finnage that might get caught in a regular net. It can also be used to scoop out small particles in the water or the foam residue that you sometimes get when you first put sand into a tank. I also use it to scoop out any undissolved particles when I mix up my saltwater.
I'll second the suggestion for a toothbrush. Good thing to have to clean ornaments or scrub off filters, powerheads, etc...
8 buckets
3 gravel vacumes of different sizes
4 nets
3 towels
dust sheets
alot of different meds including the norm, fin rot, ick, velvet, meth blue, melafix and some others
Snail away which I got given
4 tooth brushes
4 magnetic class cleaners
2 smaller tanks for quarantine ect ect
spare gravel, rocks, wood, plants
and always the hope of one day winning the lottery for my dream tanks.
A bottle of tea tree oil for Melafake, a bottle of concentrated salt solution (with dosage worked out), plastic craft canvas (for dividers, fry, etc), loads of airline tubing (for airpumps, syphons, filters, etc), some big nets (for catching hard to cath fish), frozen lancefish, frozen prawns, cucumber...