I doubt its the blocks in your pics that's causing thus.
really recommend you empty the tank, put fish in a small tank with filter and heater, they should hopefully be fine while you sort this out.
try with 1cm poly first. If that does still show an uneven tank then maybe its your floorboards that cannot cope with the weight if your tank and water.
that's worst case scenario though.
Remember 1 litre of water is about 1 kilo in weight, so am assuming your tank is about 250 litres, so that's 250 kilos plus weight of tank and decor plus weight of worktop and blocks, over 300 kilos resting in a few floorboards. Heck of a lot of weight!
Hopefully you have concrete floor or supports.
do not put a 2cm shim under the right side of rank, that's gonna add pressure to middle of your tank and may break the base glass.
lifting up a tank on one end full of water is really asking for trouble. Glass is brittle and does not bend well!!
you do not want a broken tank with 250 litres of water all over your floor with fish dying. Really not an experience even worth think about!
Ask someone to help you out and get this done properly. There are no short cuts or quick fixes am afraid.