they sell a chemical that will do it, just be careful with it. Its usually under a name like Ph Down or something, i forget if standard marine buffer does it or not.
Whatever you buy, mix it in water first dont add it straight to the tank.
is this harmful for my fish? everything seems to be ok.. i mean i read some threads elsewhere, (on freshwater) that said to leave it, but i dont know about that,
pH chemicals should be avoided in all tanks. They are usually made from phosphates, which when unbalanced lead too al sorts of problems.
Are you using tap water or distilled water / ro water? The standard salt solutions in ro water should be fairly stable and give you appropriate results. That being said make any changes slowly, especially if you fish have been in the water for awhile.