When should I add dechlorinator to my fish tank for healthy fish?

Fresh tap water can be left sitting over night and any chlorine in the water will evaporate. It can then be used for your tank. Depending on the type of fish you have you should adjust pH and hardness of the water and of course temperature. Your local pet store can help you with that.

Unless you have cleaned your tank with bleach (which you should never do) then it is probably due to the chlorine in the tap water. There are dechlorinating chemicals you can get from your pet store that should take care of that.

Reptile water conditioner usually contains chemicals such as Water, Dechloraminating Agents, Synthetic Organic Polymers (Colloids), Chelating Agents, Electrolytes. The reptile conditioner usually also has a chemical that binds calcium in the water as well as a slime protecting agent that is good for amphibians and fish. Aquarium water conditioners (for fresh water fish) contains chemicals such as sulfates such as sodium thiosulfate. Thiosulfate is a Redox reducing agent. Either should be ok for reptiles or fish, however I would stick with reptile conditioner for reptile tanks or mixed (reptile and fish) tanks and for aquaria with fish only I would use specific aquarium water conditioner. The main reason to use a water conditioner is to remove chlorine in water. This, however, can also be attained by leaving the aquarium water fresh from the tap sitting over night. The chlorine will dissipate.

Yes, tap water may contain chlorine or chloramine. Either can be toxic. Chlorine will dissipate if water is left out for 24-48 hours. This is called aging. Chloramines (used by many municipal water systems) will not dissipate and must be removed chemically with a water conditioner. StressCoat by API or NovAqua Plus by Kordon are both adequate conditioners. Look in the aquarium section of the local Petco or fish store for them.

Hi I don't recommend using distilled water in your tank as the "osmolarity" of the water can cause problems for your fish. Either pick up a bottle of water conditioner from a pet store or draw a large bucket of tap water and let it sit for 4 or 5 days. Some of the chemicals will evaporate out when left to sit, but it's not such an ideal option.

Some iron is necessary for a healthy aquarium, but too much can promote algae growth or impart an unsightly yellow or orange tinge to the water. Reverse osmosis (RO) filters are deal for removing mineral and heavy metals from water but they can be prohibitively expensive. Buying RO water from a local market or pet store is also a possibility but not economically feasible if you have a large tank. The best option may be a Polyfilter, which can be purchased in the local pet store or online. These filters will remove iron or other metals/chemicals and must be changed monthly or quarterly. You can also buy a water conditioner that will precipitate out some of the iron which can then be removed by mechanical filtration. An example of just one is Rid Metals (by Kordon). Follow label directions carefully, perform regular water changes and monitor iron levels with a home test kit or by submitting a water sample for testing.

RO water is devoid of minerals and trace elements that help stabilize pH and other parameters. If you can add these after filtration then use your RO water. Otherwise, RO is commonly used to "top off" fish tanks that lose water via evaporation.