The 18-55 lets you focus nice and close. I don't know the 50-100, but it may be good for some pics further back in the tank.
You're a little limited with ISO on the entry level cameras. I wouldn't go over 400, as the result will be very grainy and require extensive noise reduction in PS. That means that you need to use the flash on your camera for fish etc, as you won't have a fast enough shutterspeed to freeze the motion. A speedlight would help a lot here as you can angle it to reduce the shadows cast behind the fish etc. IMHO this would be a better purchase than expensive lenses. It will give you far more mileage in all of your photography.
For corals, you can switch off your pumps to keep them still, thus allowing you to use slow shutter speeds with you camera mounted on a tripod. You must switch the lens to manual focus, or it will inevitably focus on the wrong part.
If this sounds like gobbledeegook, tell me where you're stuck and we'll help from there.
Best advice that I can give, is shoot, shoot, shoot. You'll soon get that first real cracker shot, which will inspire you to shoot, shoot, shoot some more.
By all means enhance your photos in photoshop or similar. You photo should reflect what you saw.... not what your camera gave you. They'll usually need some contrast added, some sharpening, noise reduction, cropping, lightening of shadows and maybe a touch of saturation. Just always be careful not to destroy the picture with unnatural levels of any of the above.