Many modern tanks have what is called a floating base so they're not necessary. They're only needed when the bottom glass is directly touching the surface it's sat on.
I know Juwel don't recommend a polystyrene base. I made that mistake when I got mine and luckly someone on here pointed it out.
My tank is stood on a solid wood piece of furnicture and I didn't want the furnicture getting marked so there's a thick piece of polystyrene on the furnicture, then a piece of wood on top and the tank on top of that.
My Dad swears by standing his tank on polystyrene though. He says it gives it insulation and keeps the temperature stable. I've no idea if this is true or not though
I guess polystyrene would help insulate the tank in theory, though I'd imagine the amount of heat lost through the bottom where there's substrate and a stand for it to get through is much less than the sides of the tank where it's just glass.
If its not a floating tank then I would use polystyrene underneath. If not you only need the smallest bit of grit which could crack the glass base once the weight of the water, substrate, rocks, etc. Are added. Especially the larger tanks.
I have 750 kgs of water in my tank and would not even think about not using polystyrene.
yeah mines got a plastic base that the tank sits in, but i saw the sheets for sale and remember dad having them but didnt know what the benefits were, if any, good job i did buy a sheet then
yeah ive got a nice big thick piece under mine. i wouldnt of thought that much would be lost through the bottom because heat rises so more would escape through the top and sides but i suppose some could get out through the bottom. i do it so the weight is spread evenly and its less likely to crack up the pressure