So my nitrates are still creeping up slowly ~5-10 (going from ~5 to ~15) ppm a week. I'm controlling it with regular water changes, but I need to figure out the source.

All other parameters are really good.

I have 15lbs live rock

CUC:
80 Dwarf Ceriths
5 Nerites
4 Florida Ceriths
12 Nassarius
1 very small Blue Hermit (they thought he was a dwarf cerith when they mailed him)
1 Emerald Crab

BioCube 14
Chamber 1: Heater
Chamber 2: Filterfloss, purigen, chemi-pure elite.

Any ideas?

live rock may be leaching if its new maybe,
are you putting any food in the tank?

The only food that gets added is a few sinking pellets for the nassarius snails.

ro/di fo WC's?

Yup. Straight from the LFS.

Ever test the water for nitrate? Maybe it's their water.

how often? enough pellets on a daily basis could do it or possibly some snails dying off, since you have so many? those are my best guesses, i wouldn't feed the nass but maybe once a week. either way keeping up w/ the water changes like you are should eventually fix the problem as your system catches up to the load as long as you don't overfeed

I've gotten it from 2 different stores. But I haven;t tested their water. I'll do that next time.

But Bluefrog is probably right. Just gotta keep on top of my WCs and hope the system stabilizes.

You really have 80 dwarf ceriths, in addition to 21 other snails?!?!!!

Normally invertebrates have a minimal impact on bioload, but 101 snails plus a hermit and emerald in a 14 (actually smaller) gallon tank sounds like enough to show a measurable impact. I think this is the main issue.

Also, did you do any modifications, like removing the stock filtration equipment? If not, go to the all-in-one forum, and read the biocube resource thread.

Why in the world do you have such a large cleanup crew? I'd sell some of it off if I were you. That's wayyy overkill.

I know. I ordered 10 dwarfs, I got 80. I need to give the extras away.

"Chamber 2: Filterfloss, purigen, chemi-pure elite." This could be trapping waste causing it to break down and increase nitrates. People love to stuff their rear filter chambers with mech/chem media but don't realize that anything that acts as mechanical media will trap nutrients and cause elevated nitrates. If you are going to use a lot of filter media you will have to rinse it in old tank water during each weekly water change.