Levelling The Fish Tank............

If you have carpet in the room, and have managed to push the cabinet right into the corner, the back of the cabinet may be raised slightly by the carpet grippers. Pulling the tank forward a bit may help.
Saying that, you may just be unlucky and have an uneven floor.
Glad to hear you've managed to get it sorted and are ready to start cycling the tank. 

Thanks for the advice and it is good reference for the future. The floor is wood, vinyl type and is is definitely the floor running out to the centre of the room, must say its typical of the area were I live and the floors throughout the house are all a bit wonky. Concrete slab makes sense and I like that idea, maybe you could vinyl wrap it to the same colour of the cupboard ?. I don't expect any problems and I've been laying and standing on the cupboard and there is no movement whatsoever and i'm a heavy lad  , but only time will tell.