Ian the N02 level is .5 well below 1PPM , so the nitrite test is indicating they are being converted.
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.
Cheers
Ian