For the aquarist, MDF is probably best suited to "dress up" a stand, in other words to cover 2x4 studs, rather than to provide structural integrity.
When working with MDF, best to do it outside (the dust is indeed toxic, as mentioned above). I have worked on MDF with a router and there was more dust than you could ever imagine. Biscuits work well with MDF but the edges might not hold a screw very well. Pocket hole screws will work but IME you need to use the coarse thread ones. When using regular screws, I have found it best to drill the pilot hole, then use a counter sink bit on BOTH sides on the pilot hole. If you don't drill a counter sink on the "back side", the MDF can "bunch up" when you drive your screw, and the pieces won't fit well together.
As far as sealing goes, do a search on the forum for "spar" or "varnish".