Just a reminder, you can use flash. You just need to know where to position yourself. Remember that you are shooting glass and light bounces, so you cannot place the camera directly in front with flash since the light will bounce. You want to be at a 45-60 degree angle from the glass. And don't shoot with the flash too close to the aquarium. Here is an example of with flash and without. It is the same tank. But I could not control the lighting nor the time of shooting since I was shooting at a club member's home during a meeting. The flash picture has the shadow in the background.