well what kind of camera do you have? DSLR? point&shoot?
Oh I saw what you had... Let's assume a point and shoot.
Well, you have to gather all the lighting in the house and light your tank. Then put the lens of your camera on the glass, and take a picture.
CLEAN THE GLASS BEFORE DOING THIS!! Because the glass is clean, the camera lens can focus on the fish inside. You just have to aim for the right moment.
Tip: Preset your focus by holding your hand about where this fish will swim in the tank. Focus on your hand (Press the shutter button down half way) and take out your hand than wait for the fish to come.
How to photograph fish - The pro mode! [DSLR]
Well, gather your expensive equipment. You are going to need a flash, a tripod and clean glass - and if you can - a macro lens.
Take of the lid of your tank, and clean the glass. Set the flash to full mode and point it down the water from above (You could need use tripod). Set your camera to the maximum shutter speed as far as it can sync with your flash. (You'll need wireless tethering to do this, or use a
cord) Set your camera on the tripod, and focus on the fish, then shoot. You can use a large aperture which will blow out the background, but it will be hard to focus. (You can also use the prefocusing method above)
Well good luck to you all!! Ask questions if you didn't understand!