Unless you are using well water (and that has it's own issues), any water added to a tank or pond should be dechlorinated. You should also be aware that tap water can have chlorine or chloramine (compound of chlorine and ammonia). Most dechlorinators will neutralize the chlorine requardless of which is used. However, if your tap water has chloramine then you will be left with some ammonia which needs to be taken care of, either with a large healthy filter or by using one of the ammonia binding products, e.g., Amquel.
As of July 2003, OCWA uses only chlorine. The concentration is normally below 1ppm, but can vary to as high as 3ppm. You can call your water athority and ask for this information.
There are many brands of dechlorinator on the market that will do the job. Please do not use ones with additives such as aloe, slime coat enhancer, etc. If you wish to mix your own and be sure of the correct amount and freshness, here is one formula:

Obtain fish-safe aqua cultural Sodium Thiosulfate (not the penta-anhydrous form of photo hypo).

Mix 130 grams with one liter of water using a clean plastic jug.

This gives a stock solution of 130 grams/Liter, that is a 13% solution.

For a 1ppm chlorine concentration, add 2 drops of the stock solution per 1 gallon of water to be treated. One liter of your solution can treat 10,000 gallons. Over-dosage is virtually impossible.

You can make the same formula in a one gallon container by mixing 492 grams of Sodium Thiosulfate with one gallon of water.

For those interested in formulas, here is the one to calculate the amount of Sodium Thiosulfate required given gallons and mg/l chlorine:

Water to be dechlorinated (in gallons) X 0.0038 (milliliters/gallon) X concentration of chlorine in milligrams/liter (or ppm) X 3.49 (units of sodium thiosulfate/unit chlorine) = grams of sodium thiosulfate needed.