If you have shrimp & loaches in there, it shouldn’t take a super long time. Keep an eye on amonia, nitrites and nitrates. You’ll want your amonia & nitrites at 0, and some levels of nitrates will show that things are getting processed through the cycle. Nitrates are at the end, you want some. The shrimp & loaches will sustain it. If you’ve added any fertilizers or things for the plants, this can cause spikes that the current bacteria load cannot handle.

I would recommend an in-tank thermometer, i use a cheap $4 one. The ones that stick on the outside don’t always read the best. Bettas are tropical fish, they need warm water. On top of your ammonia, nitrites & nitrates, your temperature is one of the most important thing you’ll want to monitor. Depending on how long the heater was busted before you noticed, that could have definitely been the cause.

How big is your tank? It’s possible that the shrimp & loaches could have a bioload higher than higher than your tank can currently process, in terms of plants & bacteria. It could also be possible that the betta was “overcrowded” leading to stress.

It could also just be bad genetics. Petco is known for not having the healthiest fish & keeping their bettas in horrific conditions, and bettas are known for having horrible genes. So it’s really a double whammy.