You don't need to wait.... Once the tank is cleaned, you can refill it with water and add conditioner. Once it's at a good temperature, you can slowly acclimate a new fish into it.
There are many ways to acclimate (introduce) your new fish into the tank. Here is one variation:
- Fill the tank with water. Add the correct amount of conditioner.
- Be sure the water is at an acceptable temperature for your new Betta.
The water should be in the 76-83F range. About 78-80F is considered 'ideal.'
- Keep him in the store cup. Float his store cup in the tank for at least 20 min. (Longer is OK.)
- If he seems stressed, you can drape a towel around the tank.
Darkness has a calming effect on them.
- Add a SMALL amount of NEW water to the cup. (Several tablespoons, or about 1.5 ounces.)
- Let his cup float for about 15 min. (Longer is OK.)
- Repeat the above two steps, until at LEAST an hour has passed.
(I generally acclimate a new fish for several hours.)
- If the cup starts to fill too much, remove a SMALL amount of water from the cup. Discard it.
- Gently release him into the tank.