I think it is a fine idea, with the caveat that you collect from a decent location, and that you do not store it unaerated. I would not be concerned with trace elements.
I discuss it here:
The “How To†Guide to Reef Aquarium Chemistry for Beginners, Part 1: The Salt Water Itself
http://reefkeeping.com/issues/2007-03/rhf/index.php
from it:
http://reefkeeping.com/issues/2007-03/rhf/index.php#2
Natural Seawater for Coral Reef Aquaria
Natural seawater can be a fine source of water for a coral reef aquarium. Many aquarists collect it themselves if they live near an ocean. The most important factors are how pure it is when collected, and how pure it remains until used. It is often suggested to collect the water offshore to avoid run-off and other pollution sources although even offshore waters can have unwanted organisms and chemicals in them. The next best is to collect it from a rising tide from a jetty or other means, to get a bit away from shore. There can be risks to using coastal waters. Depending on the location there could be enough chemicals or pathogens to harm the tank. Also, even far from shore there may be elevated nutrients and salinity that deviates significantly from “pure†seawater.
One issue with natural seawater is that it contains suspended organic molecules, bacteria, phytoplankton and other organisms. In most cases, these will not hurt an aquarium and may actually provide food for many reef aquarium inhabitants. But if the water is stored and allowed to stagnate, the breakdown of these organic materials can produce toxic compounds such as ammonia and hydrogen sulfide. For this reason, storing natural seawater is usually preceded by filtration, and even sterilization processes in some cases. It is beyond the scope of this article to detail these procedures, but aquarists storing natural seawater should be aware of these concerns and seek out additional resources to deal with these issues. For those interested in further details, Martin Moe mentions how to treat and store seawater in his Beginner to Breeder book and also his Marine Systems book.
One word of caution is in order. Some companies collect and sell natural seawater. However, it appears that in some cases this water is not handled as well as it might be. One company, for example, has had significantly elevated lead and zinc in its water, suggesting possible exposure to metals in the collection or handling process (see analyses studies linked below). Consequently, not all “natural seawater†is the same.
