Also, use your plastic carboy for transporting water from its source, and not as a fermentation or secondary vessel. You're probably getting a minute amount of oxygen permeating through the walls of the plastic, in addition to any airborne wild yeasts and bacteria which may have been introduced when it was collected or transferred. Might want to take a sanitized wine thief and get a sample of what you have. If you can't drink it because it tastes like is should be in a malt vinegar bottle at Long John Silver's, then you might want to skip the adding yeast at bottling phase, and go straight for the sanitary sewer (and put that plastic carboy in a construction dumpster or see if someone can recycle it—you'll never make another clean beer in it, again, if it's infected.)
Here's what it should look like:
If, by some chance, it's not infected you can mix up a priming solution of boiled & cooled water, dextrose, and dry ale yeast. Determine amount of yeast as proportional to the amount of dextrose you are adding—as in, get a good gram scale and don't use much. Stir it up well, then use one of those children's medicine 10cc syringes (needle-less) to measure it out and dose each bottle before filling. Or get a corny keg setup and just force carbonate with CO2 and forget about bottling.
Andrew