Lets go back to the beginning with just the betta tanks: You had two 6.5 tanks that were being used for Bettas and was having an ammonia issue later determined to be from the substrate. When you reduced feeding, changed the substrate and switched to Spring water, you stopped having ammonia issues. You ran out of spring water and then used your tap water which has chloramine in it and you started having ammonia issues again in the betta tank with the one fish. That fish has since died but the tank that held the one Betta is showing no ammonia, nitrite or nitrate.
Am I missing anything?
The only way you are going to reduce the nitrate level in the tank with 100 ppm nitrate is to do water changes with water with no ammonia or chloramine or do a 100% water change and replace all the water with water that has no chloramine or ammonia. Waiting is not going to make the nitrate level go down on it's own unless you have something like live plants using the nitrates. Doing a 100% water change in a tank with no fish will have no effect on the biological filter unless you leave it dry long enough for the materials to dry out. This is why you have the replacement water ready before emptying the tank.
I did a 100 percent water change on the tank that was empty with no fish.
My ammonia reads 0. I used tap water, instead of spring water, so water was not the issue like I thought it was. Also nitrates read 0. I added the tap water to an empty tank. No, you are not missing anything. I did some test on the tank that has no fish, but had fish like one betta. I just added tap water to my other 6,5 gallon tank and ammonia is reading 0 for now. I will keep an eye out on the ammonia card I have in the tank.
If i start to have ammonia, then I will have to use only spring water. I am waiting to see if my ammonia stays at 0.
Also i will feed betta 2wice a day, someone here told me to do that, so betta is not starving. Tanks are ready for betta. At least I hope so. I will keep an eye out for nitrates and ammonia.