I am doing a project on aquaculture,so I am using temperature sensor, water level sensor, dissolved oxygen sensor, pH sensor and also 4 motor's and water heater is it possible for a single Arduino to run all these at the same time??
I would suggest finding sensors that work on an I2C bus. This way all the sensors are only using two pins for data. That will free up other outputs for motors, heaters that will probably be on relays or other drivers that need a single or multiple pins for each devices. As you start to plan it out, if you do run out of pins on a arduino uno, look at the Mega, as it has many more GPIO pins as well as multiple dedicated UART ports.
Bottom line, find your sensors, drivers, see what type of communication they require, and start to make a list of how many separate io pins you need. Then that will tell you which board is suitable for your project.