Got some black deposits in the tube of your Camelbak? Dark stuff in the seams? Little black spots? You've got fungus, mold, and bacterial slime inside your hydration pack. Do you really want to drink that stuff? Didn't think so.
Use a little soap if you need to. But rinse thoroughly and let the bladder "soak" with clean water for a while, unless you want to taste soap for the next month. Just use plain water for your routine cleanings. Scrub into the seams, and don't forget to clean inside the port where the hose attaches. (A persistent nasty taste can mean the plastic is degenerating and poisoning you. If an odd taste persists after soaking the bladder overnight in bicarbonate solution -- 1 teaspoon per pint -- buy a replacement.)
From time to time, use effervescent denture-cleaning tablets in the hydration-pack bladder. This does a good job of cleaning and removing germ bio-membrane deposits. Put 2 or 3 slow-acting tablets (such as Efferdent) in the hydration pack with cold water. Flip the pack every 5 minutes for 20 minutes. Let some of the fizzy mixture flow down into the tube as you do so. Let the mixture sit in the pack overnight, then rinse it out and let it air dry.
After cleaning a slimy "gross-out" hydration pack, sterilize the bladder. This slows down the regrowth of spores and germs. Put about 1/2 cup of water in the bladder and toss it in the microwave. Don't boil the water! Just heat it up, a bit at a time, until the bladder is too hot to touch. Take it out and let it sit flat, so the hot water contacts the entire inside of the bladder. Leave it until the water is cool.
But if you're cheap or only plan to clean the tube on rare occasions, here's a home-made tube cleaner: Get an old derailleur cable. (Clean it first with soap and water if it's greasy!) Slide the derailleur cable through the hose.
Now put everything back together.
If you're a water drinker, you can avoid fungus and calcium deposits by always using demineralized or distilled water. If you use a calorie-containing electrolyte solution, you're going to get critters growing in your hydration pack no matter how carefully you rinse it after each use. I suggest you buy a commercial cleaning kit. But if you want to go cheap, grab an old derailleur cable and a foam paintbrush, and make them part of your bike tool kit.