My 270 is full, no indentation at all in the polystyrene. And nice set-up BTW.
Is the tank level? Get yourself a 3' and a 6' level before you fill it with water. A lifetime of less then a level with a tank that size..........not good. And you do not want to do it after the fact.
I would not expect there to be that much compaction with good material. If it is not level now, wait until you put water in it, then you will really see how it is out.
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I'm out. 
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