It was the feeding block. Stay away from such products. They are known to cause ammonia spike. Fish are cold blood animals. They can go for 3 weeks or more without food. You were away for only a single week, they could be totally fine without anyone feeding them at all.
I don't think your beneficial bacteria is any less than before. You just need to get the remains of the feeding block to get rid of the source of ammonia.
Do you have any gravel? Vacuum the bottom of tank to get rid of any debris during your partial water change.
Keep eyes on the ammonia. If it is still not at 0ppm, keep up the daily partial water change.
By the way, how long have you had your fish?
It is a generally not recommended setup to have betta in the same fish tank as nippy fish like Cardinal Tetra.
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