A baster is great for retrieving pleco eggs and squirting them back into caves.
A long tweezer is great for planting and fishing out dead leaves.
Assorted algae cleaners including single edge razors.
Pumps for filling and emptying tanks.
And this one is no longer made by Marineland--> H.O.T. Magnum filter with the adapter that allows it to be converted into a tank vac. The water is returned to the tank. This allows me to vacuum planted tanks slowly and carefully and take as long as needed without removing any water from the tank.
My triple beam Ohaus scale. I have used or still use it to measure dry ferts for plants, ammonium chloride to feed my bio-farm when it is in use, medications in some cases, salt for blocking nitrite etc.
Bleach- for use on plants with algae, for cleaning used tanks, for sterilizing anything fish related that needs to be made safe (H tank and contents that did not save the patient).
1 and 3 ml disposable pipettes for liquid anything I need to measure- ammonia, liquid ferts, liquid meds, etc.
A metal icing spatuala I use to slip under the tail end of a small pleco I need to catch and which will not let go of the glass. Once I skip it under the body from the rear, I can flip it into the net without harming it. Young plecos are good at refusing to let go.
A big pile of old bath towels, bathmats and hand towels. Often used when I take out 10 gal. of water and then put back 15 (or more)........
But by far the most important piece of equipment here that is aquarium related is our whole house back-up generator. Extended power outages are common here. In 2011 and 2012 we had 13 days with no utility supplied electricity as the result of hurricanes. Since then outages have ranged from a few hours to a few days.