made changes in tanks.js

src/models/tanks.js

@@ -12,7 +12,7 @@ const tanksSchema = new mongoose.Schema({
     customerId: { type: String, default: null },
     tankName: { type: String, default: null },
     blockName: { type: String, default: null },
-    capacity: { type: String, default: null },
+    capacity: { type: String, default: "0" },
     typeOfWater: { type: String, default: null },
     waterlevel: { type: String, default: null },
     tankLocation: { type: String, default: null },