Nirjalaa Ekadashi is the one Ekadashi in which people does fasts without even drinking water for whole day and night. But if one wants to do this kind of fast on every Ekadashi, will it be fine? Is eating something in Ekadashi fast is mandatory like many other fasts?
In fact, Nirjal is the mandatory mentioned method. Nobody says we HAVE to eat. Anything else is less than ideal, depending on our will-power and physical strength.
Ekadashi fast in decreasing order of ideal-ness:
nirjal - not even water. but you can do vayu-bhojanam i.e. breathe air :) (actually there is tapas where you don't even breathe)
jal - only drink water, nothing else. not too much either, maybe one mouthful 3 times during the day.
madhyam (natural foods):
doodh - milk/curd etc. There is many restrictions on which type of milk is satvik and sin-free. milk you buy from the store is not allowed - because it is taken from cow without feeding its calf, or pregnant cow, or cow in heat, or first 10 days after calf is born. or it is not even cow-milk, maybe mixed with buffalo etc.
fruits/nuts - banana,orange,almonds,cashews,honey,sugar etc. - again, take in moderation. maybe 500 calories. usual food intake is around 1500-2000 calories.
adham (involves fire + salt = cooking & taste): broken rice/wheat upma, dosa, idli etc.
NO FAST :
eating usual rice/chappati, eating outside in restaurant, eating lot so that you don't feel hungry
In addition, there are rules for previous day and next day food, and sleep:
Dashami day - eat only during lunch time, no food (or light milk/fruits) at night. so that stomach is empty when ekadashi day starts
wake up before sunrise, clean teeth & tongue thoroughly, bathe, sandhya-vandanam etc.
during day - avoid sleeping, exercise, going out, watching tv, talking lot. definitely no marital union.
listen to sankirtan, pravachanam, upanyasam. chant veda, vishnu-sahasranama, stotra. do japa.
avoid sleeping throughout night
dvadashi day - after bathing, sandhya etc. - break the fast with rice before ~8.30am. there is special diet for this day because of stomach acids from fasting all day. no tamarind or chili. amla-curd, buttermilk-kozhmabu, and lemon-rasam, agathi spinach are usually served. No banana related items. also, only eating once and light milk/fruits at night. and no sleeping during day time.
More exact detailed procedures can be found on other sites, or ask elders. Doing perfect ekadashi fast even once during whole life-time may not be possible.
You can clearly feel the effects of the light satvik foods on our mood and mental clarity. Regular days we would feel sleepy in afternoon, or irritable if someone annoys us. We can realize that we are not slaves to stomach and tongue.
Ambarisha chakravarthi did this every time for 12 years, and Sudarshana chakra defended him from Durvasa rishi shaapam, who ran to Shiva, Brahma and finally Vishnu, even He couldn't protect him from His own chakra, only Ambarisha himself pardoned Durvasa.