Code from https://blog.haschek.at/post/fb08b

fishtank.ino

const int pin_all = D0;
const int pin_blue = D2;

#include <ESP8266WiFi.h>

const char* ssid = "your_wifi_name_here";
const char* password = "your_wifi_password_here";
int fade_delay = 2; //how many milliseconds between pulses (the lower, the faster the light changes)

//code starts here
int lastall = 0;
int lastblue = 0;

WiFiServer server(80);

void setup() {
  Serial.begin(115200);

  pinMode(pin_all, OUTPUT);
  pinMode(pin_blue, OUTPUT);
  setAll(0);
  setBlue(0);
  
  WifiConnect();
}

void loop() {
  WifiConnect();
  // Check if a client has connected
  WiFiClient client = server.available();
  if (!client) {
    return;
  }
  
  // Wait until the client sends some data
  //Serial.println("new client");
  while(!client.available()){
    delay(1);
  }
  
  // Read the first line of the request
  String req = client.readStringUntil('\r');
  //Serial.println(req);
  //Serial.println(req.substring(5,8));
  client.flush();
  
  // Match the request
  int val;
  if(req.substring(5,8)=="all")
  {
    String text = req.substring(9,15);
    setAll(text.toInt());
  }
  else if(req.substring(5,8)=="blu")
  {
    String text = req.substring(9,15);
    setBlue(text.toInt());
  }
  else if(req.substring(5,8)=="fad")
  {
    String text = req.substring(9,15);
    fade_delay = text.toInt();
  }
  else if (req.indexOf("getall") != -1)
  {
    client.print(lastall);
    client.stop();
    return;
  }
  else if (req.indexOf("getblue") != -1)
  {
    client.print(lastblue);
    client.stop();
    return;
  }
  else {
    //Serial.println("invalid request");
    client.print("ERR");
    client.stop();
    return;
  }
  
  client.flush();

  // Send the response to the client
  client.print("ok");
  delay(1);
  //Serial.println("Client disonnected");
  return;
}

void WifiConnect()
{
  if(WiFi.status() == WL_CONNECTED)
    return;
    
  WiFi.begin(ssid, password);

  Serial.println("(Re)connecting to wifi");
  
  while (WiFi.status() != WL_CONNECTED) {
    delay(500);
    Serial.print(".");
  }
  Serial.println("");
  Serial.println("WiFi connected");
  Serial.println(WiFi.localIP());

  // Start the server
  server.begin();
  Serial.println("Server started");
}

void setAll(int newval)
{
  if(newval<0) newval = 0;
  if(newval>1023) newval = 1023;
  
    if(newval>lastall)
    {
     for(int i=lastall;i<newval;i++)
     {
      analogWrite(pin_all,i);
      delay(fade_delay);
     }
     lastall = newval;
     return;
     
    }
    else if(newval<lastall)
    {
      for(int i=lastall;i>=newval;i--)
      {
       analogWrite(pin_all,i);
       delay(fade_delay);
      }
      lastall = newval;
     return;
    }
    
}

void setBlue(int newval)
{
  if(newval<0) newval = 0;
  if(newval>1023) newval = 1023;
  
    if(newval>lastblue)
    {
     for(int i=lastblue;i<newval;i++)
     {
      analogWrite(pin_blue,i);
      delay(fade_delay);
     }
     lastblue = newval;
     return;
     
    }
    else if(newval<lastblue)
    {
      for(int i=lastblue;i>=newval;i--)
      {
       analogWrite(pin_blue,i);
       delay(fade_delay);
      }
      lastblue = newval;
     return;
    }
}