1.How about a fog/dirty water type effect. Perhaps this can also be added to the suggetion of when a fish digs in the sand or touches some plant then it would throw dirt up in the water in that small spot. Someone suggested ink from the octopus -- this would be nice too.
2.How about particles floating in the water. This could also be used with the feeding suggetion or when a fish digs up the sand or something.
3.How about an eel that stayes in little holes in the rocks.
4.Living and moving plants and animals in the coral. (I know some of this is planned)
5.Special random events. Some one else suggested it, but I wanted to put my vote for it too -- for things really special but that occur very rarely. Perhaps a creature that is hidden in the sand almost the whole time but briefly comes out. An eel swimming to another hole? The octopus realeasing ink?
Someone suggested a Yellowheaded Jawfish. Perhaps this could be a random/rare event when it comes out.
This particular fish burrows in the sand or shells on the bottom of the tank and makes its home there. It swims around rarely, mostly it hovers right above the opening to its lair in a vertical position, and if startled will dart backwards into its hole. Info can be found at this site --
www.petwhse2.com/gillsgro...awfish.asp
and a picture at this one --
www.seasaba.com/photos/jawfish.html