Tiny,
This is my first post, but I've been on the boards for a while and am just wrapping up my first cabinet DWC grow. I've got one plant going in a tub that holds only about 1.5 gallons of water. I've always topped the resevoir with the same concentration of nutes as I used with the resevoir change. Usually about a quart or two every other day. I'm using Botanicare nutes, but that shouldn't matter for you. I never saw any nute burn, and I've grown what seems like a "monster" to me under cfls. I have changed the resevoir every seven days, and I think that's a good idea with such a small volume. Ph doesn't seem to stray much, and I just make sure that what I pour into the resevoir is right at 5.7. No problems for me doing it that way. I love DWC. Only issue has been the air wand plugging up. Live and learn. Good luck.

Mo