İs this voltage enough for the microcontroller or not
What does the datasheet for the 16F877 say?
I have not read that datasheet but assume that it is similar to the 16F887.
The 16F887 can use the 5volt supply as a reference for the ADC, or, an external supply.
If you use the 5v supply as a reference, the resolution of the ADC will be 5.00v divided by 1024bits which is 4.88mV.
This is not going to give very good control for your watertank.
You could use a much lower voltage for the ADC reference, say 100mV.
This would give a resolution of 0.097mV. This would be much better, but working at low signal levels you could have problems with noise and interference on the signal lines.
If I were doing this project, I would uses an amplifier for the signal and I would put that amplifier as close to the transducer as possible in order to minimise noise pick-up on the signal lines.
JimB