Serial.println(Serial.available());
Why are you doing that?
realh=Serial.read();
You read ONE character. Must be a pretty short tank - less than 10 cm tall.
You don't seem to have a clue that the Arduino is not waiting (and should not wait) for serial data to arrive.
Serial.print("Enter Your Minimum value (in cm)");
while(Serial.available()>0)
{
if(Serial.available()>0)
{
maxi=Serial.read();
Why are you storing the minimum value in maxi?
Serial.print("Enter Your Maximum value (in cm)");
while(Serial.available()>0)
{
if(Serial.available()>0)
{
mini=Serial.read();
Why are you storing the maximum value in mini?