Right now, it is impossible to know just how much mercury is in the fish that's available in the supermarket.
"You could for example have one piece of tuna which would have 300 times the mercury content of another piece of tuna," said Malcolm Wittenberg of Microanalytical Systems, a California chemical testing company. The firm says it has developed new technology that can test the mercury level in a particular piece of fish in less than a minute.
Once tested, that fish can be labeled at the grocery store to tell consumers how much mercury it contains.
"They will tell consumers what the average mercury level is for a particular species and what the actual level is for that this particular piece of fish," Wittenberg said.
The Food and Drug Administration says mercury labeling is not necessary, however, and is standing by its current guidelines. There is a concern that anything more could needlessly scare consumers away from what the FDA considers a healthy food.
Some consumers say their experience shows the need for better information.
Joan Davis of San Francisco said her 12-year-old son, Matthew, used to eat fish nearly every day -- from canned tuna to sushi to salmon.
But then his academic performance began to falter.
"The teachers had noticed he wasn't focusing, unable to do basic math problems, forming sentences and doing writing work," she said.
Blood tests revealed his mercury level was 12.5 times higher than what the FDA considers safe.
He is now better and his doctor says other families could easily be spared. Davis believes the public deserves more information.
"We were really shocked that something that comes off your grocery store shelf could be toxic or poisonous," she said.
"Now you can test for the mercury level, and at least people know what they're getting and it won't be a roulette game," said mercury expert Dr. Jane Hightower.
A small California grocery chain is the first to sign on with the new testing. If it boosts consumer confidence -- and seafood sales -- the company behind it believes this is the just the start.
ABC News' David Muir filed this report for "World News Tonight."