I lost 17.5 lbs last month eating only tuna

I am truly not trying to crack on OP. But she will go back to a normal pattern up eating. The body will rebel and hold onto all the calories she takes in. I agree about the water for skin and health. You can do that and not cut back so drastically. I'm not knocking tuna. It's protein. But you are undereating and shortchanging yourself of what your body needs. You can have salmon burgers (even without a bun) and add some veggies. These are low cal and full of vitamins. 

You are exercising too?! this is madness hun. You can't have enough energy for that. Flaxseed is all well but as soon as you even smell a regular meal the weight will return. .It's not healthy to crash diet. Unless I am not understanding the calories....

500 is a lot of calories?! On what planet hun. No No NO.....that is not the way nutrition works. That is not enough calories. You are starving. I know you said in another thread that you are watching your $$ regarding food. I can understand that. The college or just starting out diet. But this 17.5 lost will turn into 27.5 GAINED. Your body will go into starvation mode and will cling onto every morsel and crumb you get a whiff of. You need way more protein. Probably like a 1g per lb of weight. 42g is no where near enough. No veggies, no fruit no vitamins=does not work. 

Hey if she don't wanna listen, let her find out the hard way.

Have you looked into pescetarianism? It is a lot more balanced and healthier than what you are doing. If you want to do paleolithic then go for it but this right here is not healthy at all. The weight will come back once your body is out of the shock you have put it into.