I would honestly caution you against this. While some people may say that water fasting has a lot of benefits, there are a lot of things that can go wrong. Also, you never know how you are going to handle it psychologically.
When you fast, you are essentially starving yourself. You are not removing toxins, you are destroying your immune system and making yourself weaker. You may see some weight loss result from it, but I can guarantee you that you are going to end up putting it right back on, because that's what your body does to protect itself. Some people feel euphoric when they are doing or finishing a fast, but it is not because you are actually biologically renewed. During the fast, your brain protects your body and your mind by going into "starvation mode." You are eating away at your reserves (not as good as it sounds...), and so your brain says, "okay, we're dying here. I'm going to put all of my energy into making the body feel okay, just in case we get back to some nutrients." You are DYING, and your brain is making a last ditch effort to keep you strong enough to survive long enough to get to food. When you finish the fast, you may feel euphoric, but not because of the reasons that most fasting proponents would have you think. You are happy because you are not going to curl up in a corner and starve to death.
Fasting can harm your internal organs, especially your kidneys and liver, because they are forced to work "double time" to force stored nutrients out of your system so you can survive AND to do their normal job of breaking down proteins and other substances (toxins). Usually, especially in fasts longer than 2-3 days, finding nutrition becomes the priority so those proteins and other substances will become secondhand and you will actually STORE more toxins in your liver.
A fast of 1-2 days won't kill you, but medical professionals and scientists (or even people like me, who have just taken a few biology courses at school), can say without a doubt that anything longer can very likely do lasting damage, and at the very least it is quite unhealthy.
Then there is the psychological side of things. Depending on your personality, you may actually be able to convince yourself that fasting is better for you, and start engaging in anorexic behaviors without really thinking that you are doing something bad. Encouraged by the fast to stop eating, you will just convince yourself that you're carrying on the good work of the fast. Not true. There are plenty of physical risks that piggyback off of this, commonly anemia (which will make you feel extremely sluggish and weak, and will prevent your blood from carrying oxygen to your body, since you won't have iron/hemoglobin at normal levels), and irregular heartbeat (which can lead very easily and quickly to death).
If you are looking for something that will make you feel cleansed and renewed, I would recommend a whole and raw foods diet. Eat raw fruits and veggies, and only whole grain pasta and bread. Get protein from lean sources like chicken or turkey (light meat). Take a multivitamin and make sure it has calcium AND vitamin D (to absorb the calcium into your body). Drink lots of water and don't eat anything with sugar except for fruit, and avoid all sugar substitutes except for sugar alcohols and stevia. This will help cleanse your intestines and I guarantee will make you feel cleansed, but it is sustainable for your body because you are eating the things that it is meant to eat and you are not eating processed foods, which will literally sit and rot in the bends of your intestines. Another plus on the funny side, you will #2 a lot more. Maybe I'm weird for thinking that's awesome...
Fasting seems like a good idea because we all want a quick fix and we want something that will make us feel better, but the risks, at least for me, far outweigh any benefits I might receive. In any case, I know that you are going to do this if you want to, and I would strongly encourage you to see your doctor and inform him/her of these plans. They will discourage you most likely, but you need a medical professional to know that you are doing this in case something bad happens. Also, you might get a blood test before and after just for fun, so you can see how it affects your body chemistry. That may be enough to convince you to never try it again.
tl;dr Fasts are bad for you and usually have more negative than positive risks. Don't cleanse by starving yourself, do it by eating healthy and natural foods. If you are dead set on doing it, tell your physician because someone needs to know what's going on if something goes wrong with the fast.
Good luck!
