so sorry to hear about the fish.I just have a question.How old was the betta.If the fish wasn't all that old if was highly likely that overfeeding might have done it in.Betta fish don't move around all that much so they don't eat as much as other fish would

What size tank was the fish in? It's more likely that the drastic pH and hardness changes caused by the dissolving of the block and then the water change ultimately led to the betta's death. Fish can adjust to a wide range of pH and hardness values, provided they are gradual. A sudden change like what happened when the water was changed could easily cause shock followed by eventual death, especially if the betta was housed in a small tank as they usually are. Feeder blocks have food embedded in what amounts to plaster of paris. As the inert material dissolves into the water, it dramatically raises the pH and TDS in the water, which creates havoc for you as you try to keep the water quality stable.

Did you purify the tank water with Betta Bowl Essential?