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Last month I bought a tank off craigslist with two RES's. I knew nothing about them at the time and have been reading up some since. I quickly realized the 39 gallon tank they were in was far too small for how big they are (about 7" and 5") and have now bought them a 90 gallon tank. However, the tank was a saltwater tank. How can I make sure its safely cleaned and ready for the turtles? I want to make sure I don't hurt them or kill them by putting them into the tank without knowing what I'm doing. Also what is the best way to clean the salt out of the gravel that came with the tank?

No issues converting from saltwater to freshwater. It's the other way around that can be bad.

Just clean the tank out, wipe it down with some white vinegar, scrape off any algae, then spray it / wipe it with distilled water. Let it dry out and you're ready to go.

In case you're wondering.... going from freshwater to saltwater is dangerous because sometimes copper is used to treat sick freshwater fish. The most minute trace of copper can kill your very expensive very precious coral.