int analogPin = A3;

float val = 0;

void setup() {
  Serial.begin(9600);           
  pinMode(7, OUTPUT);
  pinMode(12, OUTPUT);
  pinMode(11, OUTPUT);
  pinMode(10, OUTPUT);
  pinMode(9, OUTPUT);
  pinMode(8, OUTPUT);
}

void loop() {
  val = analogRead(analogPin);  
  val = val*20.0/1023.0;
  Serial.print("Oxygen");
  Serial.print(val);
  Serial.println(" mg/L");
  delay(100);

  if(val<=0)
  {
    digitalWrite(8, LOW);
    digitalWrite(9, LOW);
    digitalWrite(10, LOW);
    digitalWrite(11, LOW);
    digitalWrite(12, LOW);
  }
  else if(val<=4)
  {
    digitalWrite(8, LOW);
    digitalWrite(9, LOW);
    digitalWrite(10, LOW);
    digitalWrite(11, LOW);
    digitalWrite(12, HIGH);
  }
  else if(val<=5.5)
  {
    digitalWrite(8, LOW);
    digitalWrite(9, LOW);
    digitalWrite(10, LOW);
    digitalWrite(11, LOW);
    digitalWrite(12, HIGH);
    digitalWrite(7, HIGH);
  }
 
  else if(val<=8)
  {
    digitalWrite(8, LOW);
    digitalWrite(9, LOW);
    digitalWrite(10, LOW);
    digitalWrite(11, HIGH);
    digitalWrite(12, HIGH);
    digitalWrite(7, LOW);
  }
  else if(val<=12)
  {
    digitalWrite(8, LOW);
    digitalWrite(9, LOW);
    digitalWrite(10, HIGH);
    digitalWrite(11, HIGH);
    digitalWrite(12, HIGH);
    digitalWrite(7, LOW);
  }
  else if(val<=16)
  {
    digitalWrite(8, LOW);
    digitalWrite(9, HIGH);
    digitalWrite(10, HIGH);
    digitalWrite(11, HIGH);
    digitalWrite(12, HIGH);
    digitalWrite(7, LOW);
  }  
  else if(val<=20)
  {
    digitalWrite(8, HIGH);
    digitalWrite(9, HIGH);
    digitalWrite(10, HIGH);
    digitalWrite(11, HIGH);
    digitalWrite(12, HIGH);
    digitalWrite(7, LOW);
  }

}