The frag band saws would be able to cut through some smaller rocks easily, 6” would probably be the biggest you could do.
A 14” diamond concrete cutting saw will only cut through about 5-6” of concrete. So that would be about minimum size needed to cut through most rocks in one pass though I wouldn’t recommend that option. A 7” grinder with a 7” diamond blade would be able to cut about 1/3-1/2 way through a lot of aquarium sized rocks, then flip it to get the rest. A 4 1/2” grinder with diamond blad could get through partially and then you could chip off the stuff with a hammer, then keep attacking it with the grinder till you get a flat bottom.