Question about ammonia and TAN for a small experiment

[amberc533]

Hello! I am trying to set up a small experiment for my class but I am stuck on a problem. We have three saltwater tanks, with the following measurements:

The first tank will have a pH of 7.5, an NH3 level of 0.06 mg/L, and a TAN (total ammonia nitrogen) level of 20. Just NH3 is added.
The second will have a pH of 7.2, an NH3 level of ???, and a TAN of 40. In this tank, CO2 (25 mg/L) + NH3 are added.
The third tank will have a pH of 7.2, an NH3 level of ???, and a TAN of 40. In this tank, only NH3 is added.

Basically, I am trying to figure out what NH3 levels I need to get the TAN level of 40 in the last two tanks. I'm sure I am overthinking it but I don't want to overlook anything. Any tips on where to start/how to tackle this would be great, as it's had me stuck for a bit. Thank you! :-)