Sea water near the site was found to have 7.5 million times the legal limit of radioactive iodine-131 on Saturday.
Levels had dropped to five million times the limit on Monday.
Tokyo Electric Power Co (Tepco) - the operator of the plant - injected a chemical agent known as "water glass" to solidify soil around a cracked pit, from which contaminated water had been seeping into the sea.
The damaged pit is linked to the plant's number two reactor, which had its cooling systems put out of action by the quake and tsunami of March 11.
The apparent success came as Japan imposed a legal limit on radiation levels in fish, amid fears that food was being contaminated.
On Tuesday, government chief spokesman Yukio Edano announced a legal limit of 2,000 becquerels per kilogram for radioactive iodine in seafood after double that concentration was found in fish off the nearby coast."The government has decided to temporarily adopt the same limit as for vegetables," he told a press conference.
Fukushima is not a major fishing region, and no fishing is allowed in the direct vicinity of the plant.
But experts estimate the coastal areas hit by the tsunami account for about a fifth of Japan's annual catch.
Fishermen - many of whom lost their vessels as the huge wave battered the northeastern coast - fear their industry will not recover.
"Even if the government says the fish is safe, people won't want to buy seafood from Fukushima," said Ichiro Yamagata, a fisherman who lived near the Fukushima plant.
"We probably can't fish there for several years."
Chef Seiichiro Ogawa said the fears over radiation could hurt his business.
His Tokyo restaurant is trying to get more fish from the western part of Japan, which has not been affected by the nuclear crisis.
"Japanese customers are especially sensitive to this kind of thing, so I'm worried they'll stop eating sushi," said Mr Ogawa, who has already seen his business drop 50% after foreigners stopped visiting the city after the quake.
"We need this nuclear problem to be resolved."
India announced on Tuesday that it was halting food imports from Japan for three months out of fear of radiation contamination.
Tepco said this week it was purposely dumping more than three million gallons of low-level radioactive water into the sea to make room in a storage tank for more highly contaminated water.
This water needs to be removed before workers can restore important cooling systems at the plant.