Glass tank on polystyrene?

[zenkatydid]

I have a 4ft glass tank and stand that I have just managed to get stained over the weekend (in a gorgeous rich dark brown/charcoal) and am now up to the buying gravel and plants stage (I already have the light, heater, filter, etc). However, before I fill the thing: the fish shop provided me with a 4ft sheet of polystyrene to put the tank on. Why do I need this? My stand isn't solid, it's got wooden struts/bars across the centre and along the ends that support the weight of the tank. Does the polystyrene still serve a purpose in this case?

[DepotFish]

I think its just used to make sure the tank is perfectly level and remains so.

[DeFeKt]

Not exactly.
It won't make the tank perfectly level, but it will help decrease the amount, size and force of pressure points in the tank and it's glass.
Basically, it evens out the force from the tank, not level it.
Also helps to ensure the bottom of the tank isn't on a rigid surface, which would provide cause for it to crack.

In all reality it is essential if you doubt your stand.

[zenkatydid]

But the part of the stand that the tank sits on is only a frame, it's not a solid top. So, if I lay down polystyrene, it's just going to get squashed down along the struts of the frame. Does that make sense? Do I need to purchase some sort of wooden base to go on top of my stand for the polystyrene to sit on or something?

[DeFeKt]

no, but you will definetly need the polystyrene in this case, it will even out the weight supported from the tank.
In all reality you are better off getting a flat top for it if you are going to use large amounts of rocks, which will have uneven pressure points on the bottom pane. Otherwise just the foam will be fine to even it out

Hope that made sense.