When the error is nagative how PID works?

I am going to build a simulink model that mixing a two solution of two different in some proportion so that the out flow from the third tank is same as both input of the solutions of the two tanks but i facea proble when the suppose that the outflow of first tank is 25 % of the outflow of third tank here a desire input for the first tank is 25% of the 3rd tank but the actually output of the 1st tank is greater than the outflow of 3rd tank so my error is negative and I am using the PID controller to Control this outflow when using this I got the outflow negative and i am not getting positive output so what i have to do to get positive outflow. I tried every possible combination of the P I D parameter but i cant find any solution so please help me for this in this prject i have to control level of three tank and also flowout control of these three tank