Keep in mind that with smaller tanks, changes also need to be small. With water changes or top offs, it immediately affects the whole tank quite quickly. It makes it much more necessary to do less more often, than more less often if you get my meaning. 1 gal
for a 12 gal
nano (depending on water displacement) would be close to a 10% water change. In and of itself this is not that much but if the water parameters of the new SW
vs the tank are not closely matching, the corals react negatively. Closing up and sliming would definately be an indication of this. Sliming only could simpley be a feeding response.
Be sure no matter the size of the tank, new SW
is properly aged and aerated at least a day in advance before needed. It wouldn't hurt to check the chemistry of the new SW
against the tank either. At the very least be sure salinity, pH and temp are as close as possible.