My kid fell to sleep so I set up a very quick test on our kitchen island.



First, we are remodeling so sorry for the mess in the back.



This peice of foam is 30" x 16" and 2" thick. It is the blue dow product available at Lowes.



The distance between the two concrete blocks is 25.5".



The weight of the case of water and the gallon of water is about 40 pounds. If you want to check that math, there are thirty 16.9 ounce bottles in the case and the gallon of water has, well, a gallon of water (128 ounces).



The foam was flat before the weight and is now sagging about 1/2". I have pressed down on the water and got the whole thing bouncing up and down and it has not shown signs of cracking. It has been set up for 15 minutes now.







I think the upshot here is that you have little to be worried about with your turtle. It would outgrow the tank before it could break a 12x12 piece of 2" thick extruded foam. If you put down some fiberglass mesh tape before the Drylok, grout, cement or whatever you use it will be even stronger.



I do not believe you need to extend supports into the water for your platform.




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Currently keeping:



6.10 Gonyosoma oxycephalum (Javan, mixed colors)



1.1 Philodryas baroni



1.1 Lampropeltis triangulum multistriata



1.0 Rhodesian Ridgeback

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