Wood will naturally bow under pressure, and over time, the top will sag. It could then split, and off slides your tank. 30 Litres is a lot when its spread 1cm high over your entire floor!
I personally wouldnt place a tank on a set of draws(only because if te untold damage water does when it seeps throug your floors and rots away at your woodwork for yyears), but i have put plenty of TVs on them, and the same thing happens, it sags, you open the drawer and it drops, and then you cant close the draw. You end up with a drawer permantly 1 inch open. I dunno about you, but that would drive my Mrs nuts!
Easiest way to combat it is to take off the top, and place a strut from left to right across the top. Do it twice if necessary, even spaced from front and back.
That will hold most things for years. If it should ever start to bow, you can just replace the struts.
Hope this helps..