yep, there is no doubting that 300kg would be an ample limit, even the 200kg would have taken probably twice the weight in testing, but in my view, i'd go for the strongest that i could affordI would just like to add that 300kg shelving would have been tested to a much higher weight before they settled on the 300kg safe limit, most probably a 400kg test.
that wont really matter to the weight, the 10kg of sand will simply take up the room where 10kgs (ish) of water would be, so the weight/ volume remains the sameyeah remember the stones and stuff oyu have in there will add up quite alot, in a 4ft tank you will proberly end up using 10kgs of sand or so. but all your caves and bogwood and so on