Moon's oceans drains out.

[Me]

If i place a moon around a planet, and place a moon around the planet, add oceans, then the whole ocean drains out in a matter of days, i put alot of magnetic field, but nothing work, and it happens only with moons, what happened?

[SyzygyΣE]

Smaller objects lose lighter substances like water quickly because they do not have much gravity to hold it against the star's stellar winds. There's a limit to how much magnetic fields can help. The farther you put the moon from the star, the slower it loses water. Generally, you really do need to put a small moon outside of the habitable zone if you want it to retain its water.

Iron is lost less easily than silicates, followed by water and then hydrogen.

[Me]

So what you mean is that i can't make an earth like moon around an earth like planet?

[SyzygyΣE]

Of course, you can. What I'm saying is that it is difficult or an object the mass of the moon to hold a large amount of water close to its star, even if it should have a magnetic field. An Earth-like planet far from its star with an Earth-like moon wouldn't have a big problem with water loss. If you mean "Earth-like" as in not with a similar mass to Earth but actually a planet with oceans and land—which is really only possible inside of the habitable zone—then such an object's moon won't be able to have oceans because it is too close to its star, relatively speaking.

By comparison, our moon doesn't have very much water and certainly doesn't have oceans. If you just wanted to create a moon that is made of silicates and iron, you can put it quite close to the star without too much material being lost.