Fish Tank Thinker is a kind of instructional material which provides wide range of downloadable instructional materials in the form of lesson plans, visual aids, assessment tools and video presentations which are of aquatic theme. This study aims to investigate the usefulness of an instructional material the Fish Tank Thinker Material, used as an assessment tool to determine the performance of pupils in the pre-test and post-test using the Fish Tank Thinker material. Combination of Quanti- Quali research design was adopted to determine the level of performance and significant relationship of one hundred twenty pupils, twenty pupils from each grade level, and questionnaires for teachers respondents to evaluate the efficacy of the Fish Tank Thinker material

It utilized the pre-test and post-test to determine the level of performance of the pupils. The Fish Tank Thinker is used as assessment tool in the post test to determine if performance of pupils have improved..

Mean difference was used to determine the performance level of the pupils before and after the test. Pearson Product moment co relation was used to determine the significant relationship between the pre-test and post-test mean scores of pupil respondents.

Result of the study showed Fish Tank Thinker Material proved to be of great help to enhance the knowledge and mastery level of the pupils especially in the least mastered skills in Mathematics