To let more light in open up the appeture on the lens also. I think it call the f stop.
Yes, your aperture determines the amount of light that reaches the camera's sensor. Your aperture also controls the depth of field. If you want a larger depth of field (everything in focus), you will want to increase your aperture to a higher number f/8, f/11, f/16+ If you wish to single out a detail and have a shallow background (blurred) you can do this by opening up your lens f/1.6, f/2.8, f/4
Shutter speed works in conjunction with your aperture. The longer the shutter speed the more blurred movement will become. If you can manually adjust your shutter speed, I recommend not using anything lower than 1/60 of a sec with out a tripod as it will blurred.
You generally will want to use the lowest ISO setting your camera will allow. ISO in digital cameras is yet another means to control the sensors sensitivity to light. This is done but at the cost of noise/fuzz in your photos. The lower your ISO in a good-fair environment will help keep your photos sharp. The lower your ISO is in a low light situation the more noise you will have in your photo. You can keep your ISO (noise level) lower by using a tripod.
A tripod is a great tool. I consider it an essential tool for any photo, and especially if the subject matter is one or all of: water, lighting, macro shots ...which aquariums frequently are. Tripods force you to work slower and think about composition. They will also allow you to have longer shutter times and allow for bracketing. If a tripod is either not in the budget or interest you can also use the cameras built in timer and place the camera body on a stable surface. This will allow for longer exposure times without being blurred.
A flash is another great tool. I don't recommend firing a flash straight into a piece of glass but if you can point your flash and bounce the bounce the light you will be able to bring out some great highlights and eliminate harsh shadows. If you have a basic flash and can not "bounce the light" I recommend trying to diffuse it. Google "snoot" or "diffuser".
Yet another means of reflection control is positioning your lens as close to the glass as possible. Sometimes even a cheap rubber gasket between the glass and your lens for a "seal" will help control both unwanted reflections and light.
Composition is also a vast topic and practice will vastly improve all your photos. Try to use: rule of thirds, golden section rule, balancing elements, cropping, depth and framing until you understand when it is important to break the rules. I recommend also having a colored background. It will help emphasize your subject. Use a dark background for taking a picture of a light object, or, vise-versa (to clarify, this does not necessarily mean aquarium background color... this means background of the framed photo). Lastly, leave a space in a shot in front of an actually or potentially moving fish/subject and let the subject move into place.