Wow, I am really glad i found this forum/thread!
I have been planning my own graveyard/zombie 28gal tank for about a week now.
Hopefully we can help each other
I will be using a 28gal bowfront tank that I have just pulled out of storage and cleaned up. I have some dark-blue/light green (petco brand i think) gravel that I tested with a black light. The green glows radioactive neon green when close to the light. With the distance it will be sitting at from under the hood, it will be a perfect bio-hazard green.
I may also be mixing in one or two bags of solid dark blue to mitigate the green affect in specific areas.
I want the general scene to be that of an old cemetery. Then, off to the side...
I have always loved the sand fountains, and i figure if i can find neon green fine sand (shouldnt be too hard) it will produce a really cool toxic ooze effect coming out of the barrels.
Add some green glo-fish and several of the other recommendations already mentioned in this thread and it should be awesome.
For gravestones, search for miniature gaming terrain.
I already have some gravestones from Miniatures Gaming (mordheim) that I will probably be sealing and using.