How much is 10 percent water change in a 33 gallon fish tank?
It would be 3.3g.
As long as you are established and the water is good, I would recommend 5g because it's easier to dose with conditioner and it's a nice number to multiply. You can get 5g buckets from home improvement stores pretty inexpensively.
25% would be roughly 7.5g ( one full and one half of the 5g bucket ) and 50% would be about 16g.