We have a 2.5gallon round betta tank with an air pump in the middle, no filtration. Has gravel, a few plants and a few rocks. We got a red crown? betta approx 4 weeks ago. We set up the tank, put the drops in the water like we were supposed to, put the fish in the bag into the tank to acclimate it. We got betta food (my daughter tended to just dump a bunch in, but i would fish out most of the food). I did a 100% water change a week after we got it, and it was fine. I did a 25% water change weekly until last weekend, when I did another 100% change. The fish did fine for 3 or 4 days, then started hanging out at the top of the water, decreased appetite, dull color. pH was 7, ammonia, nitrate, nitrite, etc all came back normal/fine. Came home 2 days ago, and he was dead. I re-tested everything, and it was all fine, temp was between 75/80. I didn't see any cotton patches, ulcers, or anything obvious. I completely cleaned the tank and everything in it and it has been running for 2 days now.My questions are: 1. what do you think could have killed the fish so i can avoid killing the next one? 2. how long should i wait before putting another fish in the tank?