I'm making an aquarium for the city on our server (a big building with a glass tunnel). I'm going to fill it with water and put Squid and Silverfish in it. But I thought it would be better if we had more variety. So does anyone know of a plugin for being able to spawn various peaceful fish? I've done some searching on BukkitDev but found nothing.
Take a look at the Tropicraft Mod. It comes with an entire fishing mechanic and a large variety of fishes and underwater creatures.
In addition provides Tropicraft an own Aquarium Container. This may be the kind of mod you search. But keep in mind, that the players will need a client side modification, because this is a mod and not a server side bukkit plugin.