I've made an Cave for my fishes out of a Plastic Bottle.
1. Plastic Bottle of 2L
2. Razor Blade
4. Aquarium safe Silicon
6. Aquarium safe coloured stones
1. Cut the upper portion of the Bottle with the help of either Razor blade or Scissor.
2. Make a hole big enough for the fishes to move inside on the side face of the Bottle.
3. Cover the Bottle with tape.
4. Place the stones by using Silicon.
5. Let it to be dry & cured for 72hrs.
● Here is the final Product: