It could be going a bit faster because of the size, the biofilter building up quickly, the ammonia being produced by fish (and perhaps lack of being taken up by the plants).
If the results aren't too good after 2 or so hours, you might consider doing another 10%. You have to keep the nitrite down, too. You might end up doing two 10% changes daily during cycling.

Let us know how it's going.

~Ash