Many lense choices at different price ranges which would all way better than a mobile phone camera. Focusing, metering ,sensor size are better than phone cameras, yet white balance, vibrant colors can be better from phone cameras I feel.
You need to make sure lense is compatible with the 3500 as I believe its not fully compatible with AFD and AI lenses, sa.e applies for a few non nikon lenses.
Like metering and autofocus won't work with those lenses so make sure to check compatibility before you buy.
The 18-50 3.5-5.6 thats typically comes with the camera is a good starting lense fir tank fish and coral shots. I typically advise to go with the tamron 17-50 2.8 which is way better glass and focuses closer, yet a bit more expensive.
50mm 1.8 or the more expensive 1.4 version would give very sharp fish shots as well as good closeups. Tamron 90mm 2.8 macro or the more expensive nikon 60mm 2.8 macro or the 105 2.8 are great choices.
Nikon 24-120 F4 is also a good focal length for tank shots and good quality glass.
24-70 2.8 nikon lense would be a great choice but price would be prohibitive, that is a lense id love to have.
18-200 3.5-5.6 is also a good choice.
Check what lenses your brother has and maybe try them and post some pictures maybe those would be enough for what you need.
