One of the biggest problems facing the world today is global warming. Many scientists believe that our production of carbon dioxide and other greenhouse gases contributed to the warming of the atmosphere and this effecting most living organisms on earth including microalgae. Therefore, this study aims to observe the changes in the diversity of mixed microalgae communities in different stocking densities of fish under different weather conditions within effects of environmental parameters on microalgae. Phytoplankton samples were +collected from five tanks, one tank is a control tank without fish and four tanks with different densities of fish (10, 20, 40, and 80 fishes) respectively under different weather conditions for microalgal diversity study. Weather conditions and rainfall were recorded daily. Microalgae samples were collected from fish were identified, enumerated and the diversity index determined based on Shannon, Richness, Evenness, and Simpson. There are 56 microalgal taxa found in the outdoor fish tanks belonging to 6 divisions represented by Chlorophyta (38) followed by Bacillariophyta (6), Cyanophyta (6), Dinophyta (3), Euglenophyta (2), and Crysophyta (1) in three weather conditions. The highest number of taxa recorded was 41 in tank 2 (20 fish), 39 taxa were observed in tank 3 (40 fish) followed by 34 taxa in tank 4 (80 fish), 33 taxa in tank 1 (10 fish), and 13 taxa in control tank without fish. In wet weather, the number of taxa was 25 followed by mixed weather 24 and dry weather with 22 taxa. Favorable water quality during wet weather is probably led to the highest species occurrence. Control tank without fish has 13 species represented by four microalgal divisions. Chlorophyta was the most common division in all tanks with 33 taxa in tank 1 (10 fishes), 41 taxa in tank 2 (20 fish), 39 taxa in tank 3 (40 fish), and 33 taxa in tank 4 (80 fish) in three weather conditions. The highest number of species occurred in wet weather and the lowest species occurrence in dry weather conditions. The most common and dominant species was Pandorina morum found in all tanks in different weather conditions. This study concluded that the fish stocking density and weather conditions influenced microalgal species composition and diversity.