Sulfate is the third most abundant ion in seawater by weight, at 2710 ppm, so small boosts to it are likely to go unnoticed. I discuss it here with respect to magnesium sulfate dosing:
Do-It-Yourself Magnesium Supplements for the Reef Aquarium
http://reefkeeping.com/issues/2006-07/rhf/index.php
specifically here:
http://reefkeeping.com/issues/2006-07/rhf/index.php#14
What provides the potassium in saltmixes? Wouldnt that same chemical be a safe way to raise K+?
Yes. My DIY salt mix uses potassium bromide and potassium chloride:
http://reefkeeping.com/issues/2005-11/rhf/index.php#21
An Artificial Seawater Recipe
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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.