Armend

I have problem with the radiator , its the 3th radiator who i install and its going leak , i dont know where is the problem the temperature its normal 90°c, the coolant is g12 but is dirty i think they used stop leak , and the tank of coolant its look dirty , can someone tell me what could be the problem poor quality of radiator or impurity of the coolant, and maybe and channel of the engine is occupied ?, Please help me