I just made a cart a few weeks ago to move the tank from my new 270 set-up into my house (I had to get it down a long hallway, so getting enough guys on the tank was going to be next to impossible because they couldn't get on the sides). I looked at some different casters and most of the ones I found were only built to support about 100 pounds, with the biggest ones I saw at 150 pounds. I estimated the weight of the empty tank at about 500 pounds and figured 6 wheels would work.
The problem your looking at with 125 gallons of water, the tank, stand, rock, sand, sump, etc. is, to say the least, much more significant. I'd say even a possible low-end estimate would be in the neighborhood of 1400 lbs... and as others have said that weight will be point loaded on however many wheels you use (10 or more according to their recommended load ratings, which will set you back close to $100 right there).
I also think it is a bad idea to try to move a tank with rock in it unless you would be going on a perfectly smooth surface. Getting across thresholds will inevitably result in some jostling... which could likely result in a rock slide.
Moving tanks with livestock sucks... but moving one and blowing a hole in it or having it damage your floors will suck even more.