I have several 1-2 gallon-dirt based NPT's, however, I don't use any added filtration or water movement in them....just the plants as the filter...I do have to make more frequent water changes on the smaller NPT's than my larger filtered and unfiltered NPT's....(see pics in my album
Once mature (about 3 mo
) I make at least one monthly 50-60%...plants work great as filters provided that you have enough of the right species and they are actively growing
With your plan I would recommend more stem plants and some floating plants...naja grass, rotala, ludwigia, wisteria work well in the smaller dirt based NPT's...I like water lettuce for floaters....
What kind of light do you have, age of bulb, kelvin, watts and photoperiod.