Tank bows out when full....please help

[cvrle1]

Hey,

I noticed few nights ago while setting up my tank that when its full of water, tank bows out. Now I'm not sure if this was happening before and I just haven't noticed it till now or not, so I just wanted to check if this should be happening or not. Tank is 33 gallon. When I took out a bit of water (about 5 gallons or so) everything seemd fine. Tank bowed out by aout 4 mm in the centre of the tank

Any help would be good.

TIA