My scuba instructor told me that the treading test is supposed to be done with neutral buoyancy.
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Talk to your instructor to see if he/she can or willing to "modify" you to be neutrally buoyant.
Assuming it's a PADI course this isn't true. The treading/floating test is supposed be done without any swimming aids but standards do NOT say that the student is supposed to be made neutrally buoyant. Adding positive buoyancy to someone who is naturally negative I would see as using a swimming aid and to my way of thinking that would be a standards violation. In any case if I were your instructor you would be SOL.
It is, however, in the standards that *if* a student must wear a wetsuit then they must be weighted to be neutrally buoyant so if you wanted to cut corners you could put someone who naturally very negative into a wetsuit with a properly weighted weightbelt and get them to do the tread like that. It sounds like this is what your instructor did.
Personally I prefer people to do it with no aids at all. It's a water sport and you might not be wearing a wetsuit at every moment. If you're body is negative, then your body is negative and it's in your best interest to learn how to deal with it the way it is.
R..