Look here.

There is water of a certain saltness in the sea; well, fill your tank with sea water, and keep it at that saltness by marking the height at which the water stands on the sides.

When it evaporates a little, pour in _fresh_ water from the brook till it comes up to the mark, and then it will be right, for the salt does not evaporate with the water.

Then, there's lots of sea-weed in the sea;--well, go and get one or two bits of sea-weed, and put them into your tank.

Of course the weed must be alive, and growing to little stones; or you can chip a bit off the rocks with the weed sticking to it.