I talked to Sean about this same issue with mine. I had a green one that was growing HUGE - growing several extra mouths, but then it just started to shrink when I started carbon dosing. Anyway, some of these apparently do better in higher-nutrient environments, so I'd fathom a guess that a little nitrate is good for these, like Sam said. Same with Xenia, by the way!
Also, mine won't eat anything that isn't chopped really fine. I got a small $5 no-name food processor, and I use it to pulverize a mysis/phytoplankton/amino acid mixture that I feed twice a month to my LPS and rics.
I recently moved mine lower to make room for an awesome SPS piece, and when I moved them lower in the tank, they actually got bigger. Probably just trying to absorb more light. At any rate, they look a lot better, so maybe play around with placement a bit.
Lastly, unless you're cycling your tank or are having big problems, I don't know that I'd check for ammonia really frequently. If you have a big ammonia problem after the tank is established, you'll see it in the tank inhabitants, AND you'd probably observe a nitrate spike. Since most people have some problem with nitrate creep, I'd get yourself a test for that.