How do I make sure a saltwater fish tank is safe for a freshwater fish?

Vinegar.

you should always use declorinated tap water for freshwater tanks for the minerals that it contains. There should be no residue that is gonna be harmfull to worry about. If you really want to be most safe though, you can clean the tank and equipment with white vinigar then rinse it well. The only real issue you could have is the tank sitting in storage. Any tank that has been used, drained, and stored can have the possibility to leak later on since the silicon can dry-rot. especially when coated with salt.

The vinegar suggestion is O.K., but it all may be much ado about nothing. Unless your fish is a super sensitive species, just filling and using the tank should be fine. Most freshwater types are not all that sensitive to a little residual salt. Some species benefit from a little salt in their water. One tip: Most municipal water supplies are now treated with a compound called Chloramine, it's much more stable than the old style pure chlorine systems. Chloramine requires dechlorinating chemicals which are easily purchased from local pet supply spots or on-line. Just be sure the label mentions Chloramine.

white vinegar is probably the best way, just make sure you rince the tank really well after.

As long as it is well-rinsed, you don't need to worry. A tiny amount of salt will not harm most fish, some species (but not all!) benefit from it. You should always use plain, room-temperature tap water that has had a chlorine/chloramine removing conditioner added to it. AquaSafe and Start Right are 2 good brands. Fill the tank with water and let sit on newspapers overnight (preferably not on carpet). If there is any dampness on the newspapers in the morning, the tank will have to be re-caulked or pitched, it has a leak. If you cannot afford a basic fresh-water test kit (about $20), you should not keep fish. Frequent testing is not optional, it is crucial, especially during cycling. How else will you know what is going on in the tank? Ammonia, nitrates, and nitrites are colorless and odorless. Dip strips are not accurate, either. You need a real test kit. Once you purchase it, it will last a very long time, and you can usually purchase refill chemicals as needed, which doesn't cost much. I suggest you get some beginner's aquarium keeping books at any book store or library, and familiarize yourself with what all is involved. It will really be very informative. Good luck!

I don't know why you're worried, you just rinse it out and it's fine for freshwater use. any rocks you can soak for a couple days and then rinse, same with the filter and any other equipment, just rinse and it's fine. toss out any old filter pads, but any biologic filter materials are ok to rinse and reuse. really, nothing special or hard, just rinse

fill it up outside first and make sure it doesn't leak. All you need to do is fill it up and drain it a couple of times to get all the salt out. then us a water treatment/conditioner that removes both chlorine and chloramine. or use spring water, distilled water, reverse osmosis water or rain water.