You need to ensure that all bottom edges are resting on something and spreading the weight evenly. Either fill all gaps with spacers (neatly !!) or stand whole stand on a piece of ply (18mm block ply) to spread the weight evenly. MDF doesn't take too kindly overloaded via small areas.Put the plastic levelers under tank because its in tiles in kitchen and not very level you think will ok as stand isn't totally flat on floor and resting on spacers should be fine shouldn't it ?
Probably the easiest and cheapest solution. You will obviously only need spacers in bits where tiled floor is slightly too low.youthink 10 around base 1-2cm in will be ok
Good hints here are:going to try some scapes out