when buying second hand tank, the main concern i would consider is leak. Some defects cant be spotted by naked eyes, hence buying from responsible reefer and view while tank is still running are important. However, you still need to look at the glass edge, silicon, scratches, the frame & wood structure, etc to determine the condition. A well maintained tank can be judged from your first impression. The layout and planning are one fact to consider the well thought and plan made when making the tank. As buying second hand tank, the design, etc came from the original owner or tank maker, hence unless you are good in DIYing, you might have to stick to the existing design. Do some research on those fail-safe feature and speak to the owner to unless his design, maintenance, fail - safe features. Select the one that you feel more comfortable. Having said all these, buying second hand item always carry risk, thus you have to accept the fact. Buying new will cost you a bomb and the depreciation value for tank is easily more than 50%. Hope this help.