Advice about setting up my 500 gallon acrylic aquarium

Welcome to MFK!!!
The reason some people put Styrofoam between the wood stand and the tank is to smooth out any improfections in the wood. It's just a theory but the Styrofoam most likely selflevels distributing the weight evenly across the whole sheet of plywood. That way no stress points are created if the plywood is slightly thicker in one area than another. I do it, but like I said it's mostly theory. I havnt seen any hard data to support with or with out being better.
Also you will be fine with the 3/4" ply for support under your acrylic 500.

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Thats a big tank! What are the dimensions?

That is a solid looking stand, no need to add my plywood to the top. The strength comes from the supports, not so much the top. The plywood is just there to support the acrylic fully.

I agree with what Egon had to say, it is usually recommended to use foam under rimless tanks and forgo it when setting up a rimmed tank. That says, some manufacturers have different requirements for their tanks, so it isn't always consistent. With acrylic, when in doubt I would add it.

Good looking tank, cant wait to see it set up!