and if you were to do a fast with lemon water would it work better or worse than just a water fast?

Neither lemon juice nor grapefruit will cause you to lose weight. It may appear so to some people because anything that causes you to eat less calories is also going to cause you to lose weight (if you're fasting you're going to eat less). The best way to lose weight is through healthy eating and exercise. Going on a fast may work for a day or two, but you'll probably gain the weight right back.
In response to some of the other answers, unfortunately, acid from acidic beverages does not eat away at your fat cells.

idk about weight loss, but too much lemon juice will thin out your blood, causing you to, naturally, become very ill...i dont recomend it at all

NOPE DON'T TRY IT
MY FRIEND TIRED IT SHE JUST DRANK LEMONADE TO LOOSE WEIGHT,
BUT SHE LOST HER HAIRS, THIS IS TURTH.
and it will also affect your blood circulation

You mean to survive with only lemon water? No it will not. It contains citric acid and some minerals. But surely it cannot supplement food. But since you are going to fast and want to take lemon Juice instead of water, then the Juice is better. Prepare the Juice like this. Squeeze the fruit, take the juice, add a pinch of salt , then add sugar and add lots of water. Mix thoroughly. Drink it. Lemon juice, sugar and water will help you. You can drink without the sugar also.

Don't know about lemon, but grapefruit helps. The acids eat away at some of the fat cells.

and lemon has acid

Since when did acid "eat fat cells"???? Not true at all. As a matter of fact we ALL have hydrochloric acid in our stomach for breaking down food. Why then aren't we all thin???
All those "grapefruit/lemon" water fasts do make you lose weight but it has nothing to do with what you put into the water. It's due to a lack of nutrition and calories.
The only thing lemon will do is make your water taste better. Have fun being light headed and unhealthy!

OH, wow! THis helped a lot! Lemon Juice's acids takes my fat away. Well, guess what I'll be drinking Night and Day? Yeah! ANyways, I know Orange Juice has acids...that could eat away fat. And that guy is right...lemon juice CAN take away blood, with those acids.