Will my floating shelf hold a fish tank weighing approx 33kg?
Probably. 4 brackets rated at 20kg should, in theory, carry 80kg, loaded uniformly. This depends entirely on the integrity of the fasteners and the wall, of course. The original rating of the shelf as a floating assembly isn't particularly relevant.

I'd set it up, then do a test with progressively heavier dead (non-fishy) weight. It should be apparent if it's dramatically overloaded.