i used mdf last time but ply is tougher
It's more about water tolerance, moisture resistant MDF is widely available but personally I wouldn't use it for an aquarium cabinet. You could argue that when painted etc if done correctly then there is no need to bare in mind water resilience but I don't think it's the best product for building an aquarium cabinet structure wise. A lot of MDF has become 'Caberlite' (or Cabershite as I prefer) in recent years so is not as dense either.
Marine grade ply is widely available but the price really is horrific; maybe look toward beech throughout ply instead although that is also quite expensive.
Ask the retailer to explain everything if you go that route with regard to structure, glues etc.
Although I don't have any pictures I've built an aquarium cabinet from solid wood kitchen worktops before and other furniture simply as it's really easy to work with and very tough.
This in my opinion is about the most simple way to build an aquarium cabinet; materials may cost a little more but you could have it built in a fraction of the time and requires far less work to finish.
I wouldn't recommend Pine for this above method but you could use anther wood (a hardwood) and then stain/oil/varnish to look more like your pine doors, in colour at least.
The choice of wood depends on the finish of your doors but Ash could be a good choice as could beech.
The only downfall of this method is adding doors in the same colour; easily done using a veneered plywood but depends on who you know a lot of the time.
Adding doors is easy to most types of cabinet and many cabinet hinges allow you to simply clip them on/off the cabinet instead of having to use a screwdriver so maybe worth reconsidering a hinged door.
Any questions let me know