Check out post #4 in this thread about the macro setting.
But one other trick that I didn’t mention was “setting” the white balance. I put setting in quotes because I don’t actually set anything, rather I fool the phone’s auto-balance feature. For instance, turn the camera function on, but before you aim it at your tank, aim it (in full light) at a white sheet of paper, then quickly move the phone/camera to the tank. The phone will hold its white balance setting from the white sheet while you’re focusing, etc….
Take a pic…
Depending on what you’re shooting, it may or may not look good. Try the “setting” trick again by first pointing at something else. I found that pointing at my sand, which is off-white, but bathed in blue light, is often a good middle-ground for getting the look I’m going for.
just try different initial colors and take pictures of different corals at different times a day. Won’t take long for you to figure out the combo that makes yours look best.
hope that helps!