Not all MDF is sealed, especially on the cut edges. Get a little water on the edge and it will expand and crumble. Now add in a 50lb tank sitting on top of a crumbling stand and you have trouble.
I used particle board (larger chips than MDF) as the top for my hobby desk which has 3 tanks on it, including a 29g. But I also made sure I gave it 2 good coats of paint, especially on the edges. Plus the area under the 29g is supported by two 4x4s.
Any water I spill on it gets wiped up right away.