I thought this might be helpful after all the research I did. I did this rock formation for a total cost of about $10 with a little work.
What you're going to need:
Work: 4 hours (total)
Wait: 10 days (maybe more)
1. I found the slate tile, a 25 lb piece at my local landscaping/gardening store for about $5. The aquarium sealant from my local fish store (I bought a big bottle for $15 for future projects, but you only need a small tube for $5 or so). The bucket and hammer from a hardware store.
Try to avoid multicolored slate, they usually have unhealthy metals that leach into the tank. Use solid colored slates.
2. Get out your anger: first step after you have the materials, smash away at the slate with a hammer. Pay attention to how it breaks and you can manipulate how big the pieces are.
3. After your slate is in a million pieces, pick out the pieces you want and toss the small useless ones (you'll lose about half the slate to scraps, but that's okay). Take the keepers and rinse them off, then put them in a bucket of water for about a week.
3. Change the water every day or two, then after a week, test the water after the rocks have been sitting in there for a day. Make sure the water is clear and not discolored, and check the levels, usually its the hardness you're worried about, but its not a bad idea to check everything. (Also, you might want to check the source of the water to make sure the rocks aren't changing anything)
4. If all water levels are okay, its time to dry. If you're patient, just take them out and lay them all on a towel until its all dry, or if you're like me, get a roll of paper towels and dry them off one by one.
5. Its not a bad idea to have a table you can leave covered in crap for a week. Some people say the rocks edges can cut fins, others say the fish avoid it, but I played it safe and dulled the edges (you can use a wetstone, or just a knife, or another rock even, just rub the sharp edges till its too dull to break your skin). Start arranging your rocks, I took small pieces and piled them together to make columns. Use a bit of sealant ( I used a quater sizes drop in the center and pushed the pieces together, spreading the sealant). I did not glue the long platform pieces together so I can rearrange later and it makes it easier to put in the tank, but that's up to you.
6. Let the sealant dry for at least 24 hours, though I waited 72, before putting them in. Arrange in your tank, and you're done!
1. Another trick is to make sure the slate isn't disolving. While the rock is in the water, try scratching it, there's should be no mark on the rock (unless its residue from your nails) and there's should be no bits of rock under your nails.
2. Be careful while breaking the rocks, I suggest safety glasses just in case. The pieces can be sharp, and they can go flying, so be careful.
3. You can use a piece of wood (I used a 2x4) under the tile while your breaking it to try to get bigger pieces.
4. I suggest keeping the neatly cut edges hidden to keep it looking natural.
5. Ill probably add more as I think of it, fell free to ask away!
Ps. I cannot stress enough, if the water levels are off do NOT put it in your tank. Some slate could be poisonous to your fish.