Yes, the filters must be working, the nitrite is being converted. But if you are getting a trace reading of nitrite then that may mean they are working at max and struggling to keep up with it.
It's not your main problem, but something to keep an eye on If the filters are struggling to keep up you have less margin for error if something goes wrong.
But rigging up an easier way to change water so you can do it more often is probably the best thing to do first.
How old is your test kit? Take a water sample to your LFS and have them test it. Also you could try Water lettuce or hyacinth to help suck up some NO3.