Severity Code Description Project File Line Supcannot convert from 'System.Web.Mvc.JsonRequestBehavior' to 'Newtonsoft.Json.JsonSerializerSettings'

The second parameter for Json method in Web API controller is incorrectly assigned, since ApiController Json method requires JsonSerializerSettings as second argument:

protected internal JsonResult<T> Json<T>(T content, JsonSerializerSettings serializerSettings)
{
    ......
}

The MVC controller counterpart for Json method is shown below:

protected internal JsonResult Json(object data, JsonRequestBehavior behavior)
{
   ......
}

In this case, if the controller class containing Get method above extends ApiController, you need to change 2 return Json statements to return new JsonResult as given below:

public class ControllerName : ApiController
{
    public JsonResult Get()
    {
        try
        {
            using (smartpondEntities DB = new smartpondEntities())
            {
                var pond = DB.Temperatures.OrderByDescending(x => x.WaterTemperature).FirstOrDefault();

                // return JsonResult here
                return new JsonResult()
                {
                    Data = new { success = true, sensorsdata = new { id = pond.WaterTemperature, CurrentTime = pond.CreatedDate }},
                    JsonRequestBehavior = JsonRequestBehavior.AllowGet
                };
            }
        }
        catch (Exception Ex)
        {
        }

        // return JsonResult here
        return new JsonResult()
        {
            Data = new { success = false },
            JsonRequestBehavior = JsonRequestBehavior.AllowGet
        };
    }
}

If you want to use MVC controller when returning JSON instead, change ApiController to Controller class from System.Web.Mvc namespace and keep return Json(...) there.

Similar issue:

JSON return error with ASP