Is distilled water safe for my fish tank if I lack conditioner?

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.

Creek water would not be recommended as you would likely need to heat it and the risk of waterborne pathogens (even in fast moving water) would be greatly increased. Ordinary tap water can be "aged" by leaving it out in a wide mouthed container for 24-48 hours prior to use. A water conditioner would still be recommended to minimize stress after water changes.

Water softeners leave very little salt residue in the treated water. Most, not all, fish *should* do fine in softened water. Make sure you get species that does better in soft water. Use a hardness kit and monitor KH. This parameter is what can affect pH. You can set aside a gallon or two and test it after it has aged for 24-48 hours to confirm it is safe to use. Check the softener unit for a bypass valve. Some units will have one or you can get one installed. Taps/faucets outside may not be in line with the softener, so you can use water from there if necessary.

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.

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.

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.

Water conditioning can be complicated and I recommend to stop by your local aquarium store. They are most often familiar with the local water quality and can give recommendations specific to your location. Some areas need buffering solutions to help control things like the water pH. Here are a few links which may help answer some of your questions about water condition: https://www.aqueon.com/articles/freshwater-aquarium-water-quality https://www.petcoach.co/article/aquarium-water-management-alkalinity-vs-water-hardness/ Before moving your goldfish, make sure all the conditions are appropriate for goldfish: https://www.aqueon.com/information/care-sheets/goldfish https://www.petcoach.co/article/how-to-help-your-new-goldfish-thrive/ I hope this helps and please feel free to post any additional questions.