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Could you add a second water bottle on the bottom level? As cypher says they need access to water at all times and you also need to consider when he gets older that he may not be able to go up the tube anymore so something needs sorted out soon. For now you can use a bowl of water on the bottom level. Even with a tank type cage there are ways of fixing a water bottle at the lower level such as using a free standing bottle holder or vecro to attach a bottle or you can drill or melt a hole to push the spout through so that you can attach a second bottle. Tubes get stuffy so keep emptying it and try and work out an alternative access route to the upper shelf.