This is my advice about how to prepare spawn tank for betta fish.
1. Clean your spawn tank with salt or KMnO4.
2. Add tap water into your spawn tank with 3-4 inch height.
3. Add aqua plants into your spawn tank. Please see picture.
4. Add Indian almond leaves about 2 leaves.
5. Leave it at least 2 days.
6. Now your spawn tank is ready.
7. Put the male & female betta fish into this spawn tank.