You can certainly add a lot more, but here's a basic recipe for the major and the biggest of the minor ions:
What is Seawater? by Randy Holmes-Farley - Reefkeeping.com
http://reefkeeping.com/issues/2005-11/rhf/index.php
http://reefkeeping.com/issues/2005-11/rhf/index.php#21
An Artificial Seawater Recipe
For those who are interested, the following artificial seawater recipe is taken from "Chemical Oceanography" by Frank Millero. It makes a recipe that matches 35 ppt seawater in terms of major ions, but does not try to match all minor and trace elements, most of which will be present as impurities in the major elements.
23.98 g sodium chloride
5.029 g magnesium chloride
4.01 g sodium sulfate
1.14 g calcium chloride
0.699 g potassium chloride
0.172 g sodium bicarbonate
0.100 g potassium bromide
0.0254 g boric acid
0.0143 g strontium chloride
0.0029 g sodium fluoride
Water to 1 kg total weight.