Frequent water changes are the best way - just make sure you do them in a manner not to totally stress the (already stressed!) fish too much.
Can you use 2 sponges? If so, keep on with the waterchanges as you are doing, and seed a 2nd sponge in your main tank, then put them BOTH in the QT in a week (minimum).
Can you use 2 sponges? If so, keep on with the waterchanges as you are doing, and seed a 2nd sponge in your main tank, then put them BOTH in the QT in a week (minimum).