Uncertainty of environmental parameters in an aquatic ecosystem may occur within matter of time leads to sudden fluctuations. It may happen anywhere at any time due to both natural and anthropogenic sources. The survival of the aquatic organisms in these ecosystems may directly or indirectly depends on the tolerance limit of that particular organism against the sudden change and the physiological immune response of the species. Here, an attempt has been created with an experimental run where sudden temperature and pH change were monitored. Red Bellied Pacu has shown a greater survival towards rapid fluctuations in temperature with variety of behavioral response. But when it comes to the case of rapid change in pH the fish has shown mortality with different point of pH value and got higher death rate in higher pH value. This results shows that pH has a more significance in tolerance and survival of Red Bellied Pacu when compared to temperature in different environments especially in more alkaline waters.