a top off is the addition of pure-filtered fresh water. when your tank water evaporates, only pure water does, not salt. so you have to add pure water or else your salinity will get too high and everything could die.
a refugium tank is another tank. it can be used to put something jsut in case, but it is mainly used for its biological filtration purposes. a refugium is not meant to be seen, so it is as dirty as can be. you have no hermits or snails in there. you let algae grow. the algae will help consume nitrates. same thing with macro algae. the extra live rock and sand bed are just there to add more if you already like the amount you have in your main tank. this helps break down things like amonia and nitrates because benificial bacteria grows on it. the sand bed can also absorb many toxins.
if your live rock is uncured then it can be used to cycle the tank. if it stinks, it is uncured. if it smells like the ocean, then it is cured. this means that there is nothign dead on it. only add cured live rock to a tank consisiting animals because it will cause an amonia spike. or put a few raw shrimp form your grocery store in your tank. just a few, depending on the size. i used 2-3 for my 40 gallon. or if you want your levels to not go as high when cycling which is a good idea, feed your tank every day jsut like you woudl if there were fish in there.
