my 2 cents... i've been using zeovit for almost a year i guess, colours most definately improved than without it and the water is exceptionally clearer. the colours i do get in some corals are much brighter (pink monti digitata for example) than with the same coral in others tanks (eg: puttputt's) i've stopped using zeovit as of about 3 weeks ago, purely for experimentation purposes (i want to see what happens to acro colour's without dosing) i also prefer the deeper colours that people who dont use zeovit (eg: cracker) are getting (although they dose other additives such as amino acids). since i've stopped using it, i certainly haven't lost colour however some corals haved changed in colour ever so slightly (eg: pink monti digitata is more peach coloured - which incidentally looks cool!) and some acro's (from what i can see, mainly those that were tan in colour and showing small signs of purple) are becoming purple much much faster (could just be coincidence though) whether or not zeovit retards some colours (purple?) and pulls out other colours (pink monti digitata) who knows. it has only been a few weeks so dont think the zeovit bacteria would disappear out of the system that quick though.
all in all, i cant draw any definitive conclusions yet, but i'll certianly be monitoring closely to see what happens. the only other things i've changed is increased feeding by 2-3 times more once i stopped dosing. i dont see much harm in some levels of nitrates in the system (take reef's tank for example) so long as you have effective skimming and biological filtration to remove phosphate quick enough. i believe some levels of nitrates are in fact beneficial for a reef tank? even so, nitrates and phosphates read zero (salifert) in my tank (although need a colorometer test for more detail) and corals are still gaining colour even with the large increase in feeding so im assuming i have enough filtration to export nutrients.