Stable parameters are the most important in a tank...
I know that my parameters are off whack when my aro stops eating... I test water and its usually the ph that is low one time <6.
As soon as the ph is back up to 7 my aro shows immediate appetite. Ph is linked to KH, usually low ph means low buffering capacity in the water leaving it susceptible to ph crashes.
lamsimon, as for 0 nitrates... that would leave me a little concerned. With 0 nitrates it would leave me to think that the nitrogen cycle is not complete. Have you checked for nitrites and ammonia? If they both read 0 then thats good, but you should read at least minimal indications of nitrates...
How big is your tank/what filters are you running? On my 220g, I'm running an FX5 and a 2080. I've got two 9" Oscars, an 8" Dat, a Convict and a 5" Motoro pup. I have a tough time picking up any nitrates with my Hagen test kit... Maybe we're way overfiltered or something.