The water tank that was built during [Alauddin Khalji]‘s reign (1296–1316) in the second city of Delhi to meet the water supply needs of the newly built fort at Siri, was originally known as Hauz-i-Alai after Khalji.

But Firuz Shah Tughlaq (1351–88) of the Tughlaq dynasty re–excavated the silted tank and cleared the clogged  inlet channels.
The tank was originally of about 50 ha (123.6 acres) area with dimensions of  600 m (1,968.5 ft) width and 700 m (2,296.6 ft) length with 4 m (13.1 ft) depth of water.
When built, its storage capacity at the end of each monsoon season was reported to be 0.8 Mcum.
Now the tank size has substantially reduced due to encroachment and siltation but is well maintained in its present state (pictured).

Feruz Shah who ruled from his new city called the Firozabad (now known as Feroz Shah Kotla) – the fifth city of Delhi – was an enlightened ruler.
He was known for "his keen sense of historical precedent, statements of dynastic legitimacy and the power of monumental architecture".
He is credited with construction of new monuments (several mosques and palaces) in innovative architectural styles, irrigation works and renovating/restoring old monuments such as the Qutub Minar, Sultan Ghari and Suraj Kund, and also erecting two inscribed Ashokan Pillars, which he had transported from Ambala and Meerut in Delhi.
At Hauz Khas, he raised several monuments on the southern and eastern banks of the reservoir.

Recent lake restoration efforts
In efforts made in the past by the Delhi Development Authority to develop Hauz Khas village, the inlets to the reservoir were blocked and consequently the lake had gone dry for several years.
To rectify the situation, a plan was implemented in 2004 to store storm water generated at the southern ridge of Delhi behind an embankment and then diverting it into the lake.
An outside source has also been tapped by feeding the water from the treatment plant at Sanjay Van into the lake.
Unfortunately, in spite of the plans, a mixture of partially treated and raw sewage ended up flowing into the lake creating a water body that was more similar to an oxidation pond than a lake.
The water turned green from the amount of algae that grew and a foul smell permeated around the park and surrounding areas.
Various attempts were made to remedy the problems and there were temporary minor improvements from time to time, but none were fully successful and the lake languished in this condition until 2018.
The lake finally saw a permanent change in water quality in 2019 when a citizen initiative was started by EVOLVE Engineering and the Hauz Khas Urban Wetlands were created with public donations and with the help of a corporate sponsor.
Two constructed wetlands were built, one to filter the incoming water flow and one to filter the existing water body, as well as numerous floating wetland islands that were adopted by members of the public.
Together they form the largest constructed wetland system in Delhi and are unique in that they were entirely funded by and built by individual citizens and a corporation.
Although still in progress, the project has already started operation and for the first time since its inception clean water now flows into the lake.
EVOLVE Engineering is made up of two professional engineers who are also helping the local authorities in improving their management of the lake and to ensure that the lake water quality continues to improve.
The water tank that was built during [Alauddin Khalji]‘s reign (1296–1316) in the second city of Delhi to meet the water supply needs of the newly built fort at Siri, was originally known as Hauz-i-Alai after Khalji. 
But Firuz Shah Tughlaq (1351–88) of the Tughlaq dynasty re–excavated the silted tank and cleared the clogged  inlet channels. The tank was originally of about 50 ha (123.6 acres) area with dimensions of  600 m (1,968.5 ft) width and 700 m (2,296.6 ft) length with 4 m (13.1 ft) depth of water. When built, its storage capacity at the end of each monsoon season was reported to be 0.8 Mcum. Now the tank size has substantially reduced due to encroachment and siltation but is well maintained in its present state (pictured).
Feruz Shah who ruled from his new city called the Firozabad (now known as Feroz Shah Kotla) – the fifth city of Delhi – was an enlightened ruler. He was known for "his keen sense of historical precedent, statements of dynastic legitimacy and the power of monumental architecture". He is credited with construction of new monuments (several mosques and palaces) in innovative architectural styles, irrigation works and renovating/restoring old monuments such as the Qutub Minar, Sultan Ghari and Suraj Kund, and also erecting two inscribed Ashokan Pillars, which he had transported from Ambala and Meerut in Delhi. At Hauz Khas, he raised several monuments on the southern and eastern banks of the reservoir.
Recent lake restoration efforts
In efforts made in the past by the Delhi Development Authority to develop Hauz Khas village, the inlets to the reservoir were blocked and consequently the lake had gone dry for several years. To rectify the situation, a plan was implemented in 2004 to store storm water generated at the southern ridge of Delhi behind an embankment and then diverting it into the lake. An outside source has also been tapped by feeding the water from the treatment plant at Sanjay Van into the lake. Unfortunately, in spite of the plans, a mixture of partially treated and raw sewage ended up flowing into the lake creating a water body that was more similar to an oxidation pond than a lake. The water turned green from the amount of algae that grew and a foul smell permeated around the park and surrounding areas. Various attempts were made to remedy the problems and there were temporary minor improvements from time to time, but none were fully successful and the lake languished in this condition until 2018. The lake finally saw a permanent change in water quality in 2019 when a citizen initiative was started by EVOLVE Engineering and the Hauz Khas Urban Wetlands were created with public donations and with the help of a corporate sponsor. Two constructed wetlands were built, one to filter the incoming water flow and one to filter the existing water body, as well as numerous floating wetland islands that were adopted by members of the public. Together they form the largest constructed wetland system in Delhi and are unique in that they were entirely funded by and built by individual citizens and a corporation. Although still in progress, the project has already started operation and for the first time since its inception clean water now flows into the lake. EVOLVE Engineering is made up of two professional engineers who are also helping the local authorities in improving their management of the lake and to ensure that the lake water quality continues to improve.
Hauz Khas Complex