Get a bottle of prime water conditioner, from Seachem. It detoxifies ammonia, nitrites, chlorine, chloramine, and heavy metals in your tap water so it is safe to use. It should take care of pretty much anything that is in your water supply, plus it lasts a lot longer than most other dechlorinators because it is more concentrated. What makes your water so bad? As long as it isn't something like bad heavy metal concentrations or other odd toxins, you should be ok. If you are really really worried, or you can't find prime in time for this water change, which is likely because it is fairly late on a Sunday, you can use pretty much any bottled water. I would avoid using Distilled water, though, since it doesn't have any minerals in it, and fish need those. They sell mixes like RO Rite to replenish them if you do choose to use some form of RO (reverse osmosis) water or Distilled water. Regular old spring water would be fine, but it will get expensive quickly.
