Testing apparatuses and their methods of use for testing core samples with treatment fluids, such as reactive fluids, such as acidic fluids, are provided. The testing apparatuses include a top and a base housing coupled together having a sample recess, a viewing window, and a primary distribution hole. Within the sample recess a core sample assembly is secured and immobilized. The core sample assembly in the testing apparatus is viewable through the viewing window and fluidly accessible through the primary distribution hole. Optionally, a light connector coupled to the testing apparatus provides light into the core sample assembly. Methods of using the testing apparatus include providing a testing apparatus with a core sample assembly secured and immobilized within the sample recess of the testing apparatus, introducing a treatment fluid to the core sample, and detecting the interaction within the testing apparatus of the core sample with the treatment fluid.