I get the same residue in my mixing barrel. I've used Reef crystals, IO and red sea pro. It doesnt seem to have a negative effect on my fish tank.
Found this on the H2Ocean D-D site: quoted from site
Brown deposit in H2Ocean Pro Plus Salt
Question:
I have recently bought a new tub of the ocean salt.
When mixed it is leaving a brown residuue on the bottom of the buckets.
I belong to a marine club and no of many other members who are experiancing the same problem.
Can you comfirm if the salt is contaminated of this is a harmfull substance.
Answer:
This brown residue is detailed on the instructions in the tub and is a residue of ferric oxide/hydroxide from the manufacturing process, (like Rowaphos). This product will bind to any phosphates and heavy metals in the water and allow them to precipitate out for removal via the skimmer.
The presence of the ferric deposits depends on the calcium source that we use to boost the calcium level. The best calcium source to use for incorporation and solubility into the final salt solution is the one which contains this ferric residue however as people tend not to like the brown deposits we do not generally use it. We have however had one batch in the UK, of which you must have received a drum, which did have such a residue as there was a supply issue on the normal calcium chemical.
This deposit is not harmful in any way and is actually beneficial to the final mix.
I hope this answers your query
http://www.theaquariumsolution.com/brown-deposit-h2ocean-pro-plus-salt
Also found this article
http://reefkeeping.com/issues/2005-07/rhf/
Quoted from article:
Solid Residues Remaining After Preparing Artificial Seawater
Most salt mixes leave behind a solid residue when dissolved, although the extent to which this occurs varies from brand to brand. I use Instant Ocean and rarely clean out the 44-gallon trashcan that I mix it in, so a significant residue builds up over time (Figure 1). In preparation for this article I removed some of this solid material, and found that it could be almost completely dissolved in hydrochloric acid with lots of bubbling. This demonstrates that these solids were probably calcium carbonate (CaCO3), perhaps also containing magnesium. Pure magnesium carbonate is undersaturated in seawater (which is detailed in later sections of this article) 1 and should dissolve in marine systems, so it isn't likely to be the precipitated material, although there may be significant magnesium in the calcium carbonate.