FTS are hard to do depending on how wide the tank is.
Use your portrait lens, probably a 18-55mm or similar for this shot.
I recommend that you place a clear UV lens over all of your purchased lenses as it will protect the glass on the main lens body. They are usually $20.
You will need a tripod. To get the most out of the shot, black out all the lights that are in the room including other tanks if you have them. This will cut back on any glare or reflections on the glass.
Then set the White Balance on the camera. Just take a pic(don't worry about focus or naything else, just color) and go thru your different WB options. Once you find the oine that looks the best for color match use it.
Then set the ISO at 400 and take another shot later at 800 and see which have the best look.
Then set the F-Stop to F22 for your first shot, then go down to F18 and see how that one looks. Then go up to F28 and see how that one came out. You will need to experiment to find out which one is giving the best focus thru the shot.
For Shutter speed try set it at 1/60 and then play with it up and done.
For metering the shot you will see a -2...-1...0...+1...+2, try to set it at the -1 and see if it is too dark. If it is then got to the 0. If it is too light then shoot it at -1.5
You may have to try some different things.
Shoot at least one Auto shot to see if the Auto program actually makes a decent pic.
It is common at a lower shutter speed that the fish will be blurred. To fix this you will need to increase shutter speed and use a off shoe flash aiming down into the tank from above.
To get a good shot it may take 10-30 exposures and changes in your settings until you find something that works.
Shoot straight on at the tank, but don't flash straight on. If you do not have a flash that can be aimed from the top down, then turn flash off.
I am still an ameatuer so others may have better advice.
www.aquatic-photography.com is also an excellent camera advice site.
When you process these shots on your computer make sure you crop them before you do any editing.
HTH, Rich