Canister filter... not entirely useless, but a very expensive way to increase flow.
217 different PVC fittings that weren't quite the fitting I needed. That and the many gallons of gas used to go back and forth to the hardware store.
Cheap test kit. Waste... of... money.
Lot's of different fish foods that my fish don't even recognize as food. The stuff I make myself is cheaper, and the fish really like it. All you need a blender and a supply of fresh sea food.
Live sand. More money I could have used for just about anything else. Your tank will have plenty of bacteria in it if you just give it the time it needs to get going. A couple pieces of live rock will have more bacteria in it than all the bags of live sand you could fit in your tank. Waste of money... I did it... but wish I hadn't.
There should be a sister thread for things we bought and never regreted. Mop, buckets, cleaning cloth (for cars, but works on glass tanks too) are high on my list.
Maybe a thread for things we want to buy... I'd have an RO/DI unit and an autotopoff system.