I prefer an actual digital camera for taking fish pictures. You normally have a better lens and more options than a phone does. Although some of the more expensive phones are pretty good too.
Clean the glass inside and out the day before you take pictures or at least a few hours before taking them.
Do a big water change and gravel clean the substrate the day before you take pictures.
Clean the filter the day before you take pictures.
These 3 things give you a clean tank and glass and are the starting point.
If you use a digital camera (not a phone) try to set it to shutter priority. You want a shutter speed of around 1/150-1/200 of a second. You want the flash on.
Some cameras and phones will slow the shutter speed down to 1/60 of a second (or less) when using the flash and that screws up the pictures due to camera shake (you get blurry pictures). Having the faster shutter speed (1/200 second) will give you clear pictures unless you are moving your hand while taking them.
If you can set the ISO, have it on 64 or 100, maybe 200 but no higher. The higher the ISO number, the less light the camera needs but the grainier the picture is and the worse it looks. Cameras and phones regularly set the ISO to 400 or more when using a flash and that is bad.
Don't try to get the fish to fill up the entire image. Just make sure the fish is central in the picture and you can crop it down if you need to.
Don't zoom in too much either. Digital cameras and mobile phones have two types of zoom (besides the actual lens that might be able to move in and out a bit). I can't remember what both types of zoom are called by one is digital and the other is whatever. But if you zoom out a lot, the camera simply crops the image and the more you zoom out, the higher the ISO number goes (unless it's set by you). Zooming out also increases camera shake (blurry pictures) and you need faster shutter speeds to compensate for that. However, the faster the shutter speed, the less light getting onto the sensor in the camera.
Have lights on above the tank and open the curtains and turn the room lights on. The brighter the area that is being photographed, the better the picture. Cameras need lots of light to take decent pictures.
Don't wear light coloured clothes when taking fish pictures because they often reflect off the glass and appear as shadows in the picture. Dark clothes are better or stand further back from the tank.
Have the came angled slightly downward and aim at the head of the fish. You want the fish swimming towards the camera and focus on the head and front half of the body. The scales on fish reflect light best and show the best colour when the light is hitting them from the front or side, not from behind.
Most professional photographers take 100s of pictures and keep a few for printing. Digital cameras are great for taking pictures because you can literally take hundreds of images and check them on your computer, keep what you like and delete the rest, then take more. All you need to do is format the memory card after each use and recharge the batteries. It's a lot cheaper and quicker than the old days of film. Then practice, practice, practice.