Unfortunately we don't yet support redeclares in the OMEdit dialogs  so you will have to write them it directly in the code.
I'm working on a solution which should be available soon. See:
https://trac.openmodelica.org/OpenModelica/ticket/2079

THANK YOU , where is  language specification? is it norrmally openmodelica guide?

What do you mean? If you're asking for the Modelica Language specification is here: https://www.modelica.org/documents/

I don't understand  how to intervene in my case, reading your link I don't found an understandable solution

You can read about redeclares in the Modelica Specification 3.x.

In your case you need to go into the Modelica source code for your model and for component tank1 just add:
redeclare Medium=Modelica.Media.Water.StandardWater to your components.

tank1(redeclare package Medium=Modelica.Media.Water.StandardWater);

You will need to add it for all the components that need it.

adrpo wrote:

What do you mean? If you're asking for the Modelica Language specification is here: https://www.modelica.org/documents/

thank you

a model that works good alone

model tanks
  import Modelica.Fluid;
  replaceable package Medium = Modelica.Media.Water.ConstantPropertyLiquidWater constrainedby Modelica.Media.Interfaces.PartialMedium "Medium in the component" annotation(choicesAllMatching = true);
  final Modelica.Fluid.Vessels.OpenTank tank1(redeclare package Medium = Medium, height = 80, crossArea = 20, nPorts = 1, level_start = 70, portsData = {Modelica.Fluid.Vessels.BaseClasses.VesselPortsData(diameter = 0.1)}) annotation(Placement(visible = true, transformation(origin = {-60, 60}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  final Modelica.Fluid.Vessels.OpenTank tank2(redeclare package Medium = Medium, height = 80, crossArea = 20, nPorts = 1, level_start = 0.0, portsData = {Modelica.Fluid.Vessels.BaseClasses.VesselPortsData(diameter = 0.1)}) annotation(Placement(visible = true, transformation(origin = {0, 20}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
  Modelica.Fluid.Pipes.StaticPipe pipe(redeclare package Medium = Medium, allowFlowReversal = true, length = 2, diameter = 0.5, height_ab = 0) annotation(Placement(visible = true, transformation(origin = {-35, 15}, extent = {{-5, -5}, {5, 5}}, rotation = 0)));
  inner Modelica.Fluid.System system annotation(Placement(visible = true, transformation(origin = {0, -20}, extent = {{-10, -10}, {10, 10}}, rotation = 0)));
equation
  connect(tank1.ports[1], pipe.port_a) annotation(Line(points = {{-60, 50}, {-40.1478, 50}, {-40.1478, 15.0246}, {-40.1478, 15.0246}}));
  connect(pipe.port_b, tank2.ports[1]) annotation(Line(points = {{-30, 15}, {-2.70936, 15}, {-2.70936, 9.85222}, {-2.70936, 9.85222}}));
  annotation(Icon(coordinateSystem(extent = {{-100, -100}, {100, 100}}, preserveAspectRatio = true, initialScale = 0.1, grid = {2, 2})), Diagram(coordinateSystem(extent = {{-100, -100}, {100, 100}}, preserveAspectRatio = true, initialScale = 0.1, grid = {2, 2})));
end tanks;

i understand now, in every component/class parentesi i put this: redeclare package Medium = Medium, and the entire model work good