This is my first Instructable and hope I did ok.
Step 1: What You Need
Empty CD Spindle( any size you want). I used one from a 50 count.
Aquarium Safe rocks or stones. I got my stones from bluffs along the Illinois river. I had them in the aquarium already, but just as decorations along the bottom. If you get your stones from nature, make sure you clean them very well. I even boiled mine before placing them into the aquarium.
Drill with small bit
Glue Gun or Aquarium Sealant
Any other decorations you want to put on the cave
Step 2: Cut Holes
Step 3: Drill Holes
Just drill as many small holes around the center as you would like, making sure you do put some holes in the spindle itself. I almost forgot that part and would not have had a very good bubble effect if I would not have done it.
Step 4: Placing the Stones
This is the fun part and does take a little time. All you need to do is place the stones with hot glue or sealant where you want them. I filled in the gaps with rocks I took from the aquarium. If the stones are too big, you can break them with a hammer.
If you made the spindle with holes in the top for a bubble effect, don't forget to leave a space in the top center for the air to escape.
Step 5: Finish Up
Place the cave in the aquarium and put some rocks around the bottom and inside and hope you fish enjoy it.