I'm not sure how different the settings are on my camera is from yours, I have the D3200, but i'm assuming its going to be relatively the same since you just have a newer model. I'll give you my current settings to at least give you a starting point..
Aperture: f5.6
Exposure: between 1/80-1/320 (Depending on the lighting in the tank and what i'm shooting I have to adjust accordingly)
Image Quality: RAW
ISO sensitivity: 1600
White balance: Auto
Those are the settings that i've currently been happy with, although i'm constantly adjusting. Fish always come out better when shot with the flash on, and because I haven't found a setting in the white balance that I like yet, I always adjust it when i'm editing. I use lightroom for that. I hope that helps and what you were looking for, but if you have any other questions i'd be happy to help.