I've been making 3D shadow plays using polarized light for a few years, but I've always wondered if the technique would work through water, across a fish tank. The answer is yes and its beautiful; moving shadow fish appear to swim the air in front of the tank. It's a powerful optical effect and is certain to amaze your friends.

Step 1: Parts and Tools

To make a 3D Fish Tank you need:

- Two or more pairs of circularly polarised cinema 3D glass (linear polarising glasses also work too)

- Two small LED torch. I recommend the Q5 Cree AA torch with front lens.

- Two magnetic fish tank cleaners

- All Purpose Adhesive (UHU glue works well)

- Strong Sticky Tape (I use Gorilla Tape)

- A piece of float glass with ground edges, smaller than your front panel of your fish tank.

- And a square or rectangle glass fish tank (with fish!).

An the following tools:

- Scissors

- Toothpick

- Sand Blaster (more about that in step 2)

Step 2: Making the Screen

For the optics to work, we need to make a special type of projection screen -- frosted glass. You have two options here. Either you can buy a sheet of frosted glass, ask you local glass suppliers if they can often order it in. Or you can find a sandblaster cabinet and make it yourself. The second option is much more fun (if you can find a machine locally).

Choose a piece of glass will fit neatly in front of you fish tank, it shouldn't need to be too big. I used the front glass panal from an old flood light, which is about 6" by 8" and works great. Don't use glass with sharp edges or glass less than 3mm thick. Once you're found your glass, it's time to frost.

To find a sandblaster, try your local glass supplier or creative glass enthusiasts in your area.

Once you find the right sandblasting machine follow the manufactures instructions. Point the nossle at the glass about 4" away from the glass and frost the glass until it's smoothy white across one surface (leave the other side shiny) and keep sanding until you can't see through the frosted surface.

Step 3: Modify the Torches

We need to remove the lens and add a polarising filter to each torch.

If the torches have small lens we will need to remove these by unscrewing the top of the torches. In place of the torch we need to add the polarising filter. The filter comes form one of the 3D glasses. Carefully break open the glasses to remove the two plastic film filters. Now we need to think about how to align the filter. The principle is the when we wear the glasses, our left eye will see light from one torch, and our right eye sees light from the other torch.

Wear the other pair of 3D glasses. Close one eye look at the first filter from either direction, try the other eye if it does go completely black. Do the same with the other filter for the other eye. Now we will be able to tell which direction you need to add the filter into the torch to block out all the light for one of your eyes. It might try a be of trial and error to get the directions right.

Mount the filters into the torches either by glueing and cutting away the excess or clamping a carefully cut filter within the torch.

Step 4: Mount the 3D Light

I used to magnetic fish tank cleaner to hold the two lights. I taped two lights together and taped between two magnetic cleaner such that they point into the fish tank. The magnetic cleaners are great way to position the light into the right spot.

Step 5: Prepare the Fish Tank

Secure the frosted screen to the front of the fish tank using the gorilla tape. Position the light on the other side of the tank and aim them so them point towards the screen. You might want to move any plants to create a good composition for the shadows.