I would split whatever biological filtration you have (live rock, etc.) between the tanks and split your critters (or if you have minimal fish, put most of the filtration into the one with the fish, as your corals don't produce much waste). If you have sand in the 30g, I would leave it be, and don't split it or disturb it and try to move it over to the 10's, but it depends on how long its been running, and how much detritus might be in it. You shouldn't go through another cycle if the 30g was well established, and you are moving some filtration over into the two 10's. 30g tank is pretty cheap, so go get a new one quick (or upgrade!) and you won't need to worry about water too much. Top off manually for a day or two until you get a new tank. Do you have two heaters? If your fish are healthy, you can also get away with no, or very light feeding for a day or two until you get a new tank. Corals can also do without light for a day or two without too much trouble. If you regularly use Prime, or at all, does that mean you don't have a RODI unit? If not, time to get one : )