The Monastery of Zoodochos Pege ( Ζωοδόχος Πηγή) was an important early Byzantine monastery famous for its holy spring (hagiasma). It was located outside Theodosian Walls, near the Gate of Selymbria of Constantinople. The site is associated with a large number of miracle stories and legends. It had a source of water or spring (pege) that came to be regarded as miraculous. Many miracles were were also attributed to mud from the spring. In the Ottoman era, this became known as Balıklı Kilisesi  (the “Fish Church”) due to the fish in the spring.
The origins of the church are unclear, though it certainly existed by the 6th century. Procopius records that Justinian (527-565) built a church dedicated to the Virgin Mary here. Other traditions state that there was already a small shrine on the site and Justinian came across it while hunting. When learning of the healing water of the site (which cured his urinary infection), he then ordered a large church to be built from the surplus materials from Hagia Sophia. 
Another tradition claimed that Leo I (457-474) founded it before he was emperor. When he was still merely a soldier, Leo I met a blind man on the road, begging for water. Leo guided him until they reached a marsh filled with mud. Leo tried in vain to find fresh water, until a voice of Theotokos called out and assured him there was water. Theotokos then told him to rub mud over the blind man’s eyes, which restored his sight. It is possible that Leo built a small shrine on the site, over which Justinian later built a large church.
In addition to the church, there was a hagiasma, which consisted of a subterranean structure with a nave was three times as long as it was wide and a dome raised above arches. From either side, marble stairs led down to the spring, set within a space said to be about twelve feet wide. This sounds quite similar to what can be found today. The miraculous spring water flowed into a marble basin accessible by staircases inside the church.
It was renovated several times in the Middle Byzantine Era. Empress Eirene, with her son Constantine VI, reportedly restored the church at Pege. After being seriously damaged by an earthquake in 869, Basil I rebuilt the church and decorated it with a cycle of mosaics. Burned by Tsar Symeon of Bulgaria in 924, the church was soon repaired and a palace was built near the church. The church was the focus of the imperial procession on Ascension Day during this era as well. 
Early in the Palaiologan era, the epitaph Zoodochos Pege was first applied to the Virgin of the Spring and a new iconography developed, perhaps based on a mosaic in its hagiasma. A mosaic image at Pege empowered the spring and the spring, in turn, empowered the icon of Zoodochos Pege. During this period it became a major site of pilgrimage and a feast day of Zoodochos Pege was instituted on the Friday of Bright Week (the Friday following Easter). The monastery underwent a period of revival during the reign of Andronikos II Palaiologos (1282-1328). This revival followed a period of decline caused by the Latin occupation of Constantinople following the Fourth Crusade. During this time, it is said the spring at Pege lost its miraculous powers. This only changed when Andronikos II reversed his father’s attempts to unite the Catholic and Orthodox Churches. This period marked a resurgence of miracles at the shrine. A large number of miracles were recorded by Xanthopoulos. In addition, Manuel Philes and others composed epigrams on the church and its paintings. Later the waters were said to cure emperor Andronikos III from a serious illness in 1330. 
In 1422 Sultan Murad II made it his headquarters while besieging Constantinople. Following the conquest of Constantinople in 1453, the church later disappeared, though the spring continued to be visited by a small number of pilgrims. It was rebuilt only in the 18th century. The current building was built in 1833 under the Patriarch Constantius I. In 1837 the monastery was renamed Holy Hospital Monastery of Zoodochos Pege and funded the building of a new hospital, outside the walls.
A legend dates to the Ottoman period about a half-fried fish that jumped into the spring. As the story goes, a monk was frying fish that leap back into the spring half-fried when Constantinople fell to the Ottomans. Their descendants, which had different colors on each side, continued to be attested for centuries. The existence of fish in the spring, along with the legend, gave rise to its Turkish name, Balıklı  Kilisesi – the Fish Church.