I would not use masonite or MDF for an aquarium. No matter how well you think you've sealed it, sooner or later water is going to get at it and it will swell and then start to come apart.
while that is a concern. others have used masonite. I don't see a reason to completely ignore it as an option. I'm sure if you want you could easily cover it with a few layers of varnish or paint, or even drylok if you want to go that far.
Though, I'd much rather prefer a G1S graded plywood if it'll be visible though.