foam compression

[untamed]

That's not right. It sounds to me like you've used very soft foam of some sort. I used 1/2" foam insulation from Home Depot. It comes in pink or blue and it should not compress under all that weight.

Please provide some details on the dimensions of the tank and the type of foam you used.

To give you some perspective, my tank is 6' x 4', about 400 gallons. The fact that it is acrylic and not glass is not significant compared to the weight of the water, rock and sand that is in it. It likely weighs about 4000lbs. The foam under my tank has not compressed in any measurable way.