Calling them what can do sums!

[bennyboy]

^ Yup, I'd be very cautious.
I'm lucky enough to have a pretty big tank that I got in a deal eons ago, at my old house, it was upstairs so I made sure it was sitting across the joists...
At my place, it's downstairs so on a concrete floor, but when I had to move it last year to re-carpet, I realised how stupidly heavy it is, even with all the water out
One tip - I have a sheet of thin polystyrene under the base of the tank, this is for the same reason - if you have a tiny imperfection/piece of grit between the two surfaces, the pressure on that one point will be pretty damn huge and can cause the glass to blow. Not good.

[andypont]

I'm not sure about the 'stilleto effect'

How so?

With the current rubber feet, Paul has a cabinet to floor contact surface of 12 sq/inches (assuming 4 rubber feet). Lets take 100kg for ease of numbers = 220lbs. 220/12 = 18.3 pounds per square inch of force on the floor.

With castors, you have just the very edge touching the floor, and it is unlikely to even amount to 1 sq/inch between all 4 of them, but even using 1 sq/inch, that's now 220psi being exerted onto the floor.

I understand the theory of small contact areas, but what I meant was I didn't know what effect that would be liable to have on the structural integrity of the cabinet, or what flooring would be under increased load. Aplogies if I didnt explain myself very well.