The present work aimed at investigating the interactions between the water quality parameters and the abundance of cyano bacteria in tilapia fish pond during the rearing period. The study was conducted at a fish farm (1.5 acre) located at the eastern shore of Qarun lake –Sinnoris – El Fayoum Governorate-Egypt. Water quality parameters and algae load were measured monthly for seven months. Water temperatures were between 21°C and 29°C along the stocking season. pH of water in the selected pond was recorded between neutral and slightly alkaline (7.25 and 8.4). dissolved oxygen (DO) recorded values were between 4.24 and 8.66 mg/l. Biological oxygen demand (BOD) was between 3.2 and 5.2 mg/l. The pond water showe d low transparency between 9.5 cm and 13.5 cm. Water salinity values go through 2.7 and 6.3 PPT. Nitrate concentrations were between 2.17 and 4.845mg/l. The total ammonia ranged from 1.0 to 4.6 mg/l, while the Phosphate levels ranged between (0.39 and 1.07 mg/l). Along the monthly examination of the fish pond water from April to October a total number of 32 genera of algae were identified which varied according to sampling time. The highest algal count number was in October (43600 org. /ml) and the lowest was in June (5394 org. /ml). The average relative density of cyanobacteria to total phytoplankton was 2.5 to 11.3%. October exhibited the highest values of blue green algae counting 1067 org./ml while the lowest values were estimated on September (400 org. /ml). Liver and muscle tissue showed pathological changes in response to bad water quality as liver showed congestion in portal area and vein with necrosis in the pancreatic acini and dilatation in the central vein with inflammatory cells infiltration, w hile musculature showed focal aggregation of dead neutrophils and focal area of fibrosis. It was concluded that there was great correlation between different water physio-chemical parameters and algae growth in general and with cyanobacterial load with subsequent effect on general fish health.