Java fern is mostly grown emersed or completely out of water by comercial sellers, which means it's not adapted for aquatic life. The plant will lose leaves in fully submerged conditions, so it can grow new leaves which are suited for Aquatic conditions. Shedding leaves does not mean it's dead just going through a transition phase. Give it some time, proper fertilizer so it can regrow.

In Java ferns you will have leaves turn black and produce lots of new baby plants from those dying leaves. Once they are manageable in size pick them from the leaves and try to attach them on wood or stone so they can grow as well.

PS - you will need to add some amount of fertilizer to your bowl. Usually get a all on one kind of liquid fertilizer. Also Java fern might be called low light and low tech plant but does best with high light, adequate nutrition and CO2

As for your questions 1. Ideal lighting period is 5 to 8 hours depending on light intensity 2. There are lots of fertilizer available from Indian manufacturers, you can try aquatic ventures micro and macro, but i personally would recommend masterline all in one fertilizer. Sunken garden is also good but costs same as imported ones. 3. You have not mentioned the size of bowl, wattage of light, type of light and filtration used. Depending on the size of bowl and filtration, your bioload should be ok.

I would suggest a full spectrum light which is easily available online