okay thanks for the tip!!
Today i got two redclaw crabs and started slowly converting it to brackish.
Good part: The crab will be healthier and live longer.
Bad part: If i ever add more fish or crab i have to put them in a seperate tank and slowly adjust the specific gravity of the water untill its safe to add them.
oh and salt creap i dont have any yet but im not looking forward to it.