Your tank does look really nice. Since you are using the phone's camera, it will simply meter the light around and dictate what it feels should be the best exposure settings. with my DSLR, I can choose to underexpose the shot, and usually am able to minimize the overexposed areas. You can also try editing software to see if you can adjust it a bit.
Your tank is beautiful! If it were me, I would search for a free HDR app for your iphone. These are not true HDR photos but it might get you closer to the look you want-- lightening the shadows and darkening the highlights. You just need to find a way to keep the phone as still as possible (resting it on an object works) while it takes the three (or however many) images.
Alternatively, you can adjust your photos in an editing program, or even intentionally bracket three photos and take them into Photoshop to combine them if you have Photoshop.
Yep more light from above, turn off flash, turn off all lights in room, stabilize phone with a monkey grip....also do not use zoom feature on phone. Thats as good as its gonna get with an iphone. Good luck