Thank you for your question. Yes, the Seachem Acid Buffer and pH Buffer are safe for fish and invertebrates. Here is the manufacturer's response about using Acid Buffer:
All of our buffers, including Acid Buffer, are completely safe for fish & invertebrates when used as directed. The amount that Acid Buffer will lower pH depends upon the alkalinity of the pond, so the answer is not the same across the board. For example, if the pond has a good alkalinity, then the pH will not drop as quickly as in a pond with little to no alkalinity. Acid Buffer works to reduce pH by converting alkalinity into usable CO2, so it does have the potential to lower pH by quite a bit in a pond with little to no alkalinity. I would recommend adding about half the recommended dose initially, allow the water to circulate fully throughout the pond, and then test pH (maybe after about 10 minutes or so). Once you see how low the pH drops, then they can add the rest of the dose. We only recommend shifting the pH by about .5 units within a 24-hour time period.