Originally Posted by thegameksk
That makes no sense though b/c I did cycle my tank originally.
Yeah it does.
a) You unintentionally destroyed the filter bacteria by rinsing the media in chlorinated water, priming the filter with chlorinated water, letting the filter dry out or turning it off for too long (perhaps a power outage in the middle of the night) or changing the filter cartridge if it's the only media. Whenever the filter is shut off bacteria only survive for a few hours without a constant influx of food and oxygen.
b) You did that stupid, ill-advised petco version of a cycle where they tell you to let the water sit for 24 hours before adding fish. The staff there are a joke :(
c) A chemical (medication, spray cleaner, a drip, etc) entered the tank and did the bacteria in and perhaps your betta as well.
d) The infamous mini-cycle, due to reason a. Somehow a large portion of the bacteria died off and there is not enough to manage the ammonia.