Been a long time since I made my living with a camera, and when I did, “digital” meant a watch with a number display, but no hands moving around a clock face.
That said, some basics:
Apeture: also caled “f-stop”. This is the size of the opening in the shutter, and controls both the amount of light hitting the film (or whatever digital cameras use instead of film–memory space?) and the “Depth of Field”–ie, how much area will be in sharp focus. Small number = big opening, but little depth. Big Number = small opening but lots of depth.
Shutter speed: how fast the shutter opens and closes each time you trigger the camera. Put another way, this is the amount of time light will hit the image recording media, be it film or memory. For shooting a fishtank, you don’t really need a fast shutter speed–the tank ain’t moving, and you don’t need to stop action. Numbers are a fraction of a second–ie, 500 is 1/500 second.
ISO–did not exist, or if it did, the term wasn’t widely used “back in the day”. We called the same concept “ASA”. This is a “grade” of film or other media that indicates how sensitive it is to light. High number indicates “faster” film–ie, the film is more sensitive to light.
Old photog’s rule of thumb–F8 @ 500 with 400 ASA(iso) is good for shooting human action (say running as in a sporting event) on a bright sunny day. F11 @ 250 would produce similar results (smaller opening but longer time) in terms of amount of light reaching the media.
For shooting a fish tank, I would avoid using a flash at all costs. Flashes reflect off shiny surfaces…like glass, or mirrors. You don’t need huge depth of field here…I doubt the tank is gonna be over 3 feet “deep” (meaning front to back from the camera) and 6 feet wide. F5.6, maybe even F4 is likely plenty of depth of field. Already said the shutter speed doesn’t need to be a big number–that tank ain’t going anywhere. If you’re hand holding the camera, you’re prolly good using shutter speeds as low as 125. Get much slower than that, and you either need real steady hands or a tripod to avoid blurring the image.
Sooo…how much light we got to work with here? Normal indoor room lighting? Darkened room, but interior tank lighting? Darkened room with no interior lighting? All that stuff matters. If you have some way of posting a shot you’ve already taken, maybe you could get suggestions on how to improve the image quality…