Hi Brian,
you are right Baking Powder and Baking Soda are different and also used under different conditions. An extract I found explains that
Baking soda is pure
sodium bicarbonate whilst Baking powder contains sodium bicarbonate, but it includes the acidifying agent, and also a drying agent (usually starch).
First of all try to use Baking Soda and try using a measure of a tablespoon of baking soda per 25 litres and after a day check the ph if it reaches around 8.2. Now if I where you I would half this dose and see if the makes an effect. Our Water is already hard on it's own so it could be that you need to identify the right dosage. Do this slowly and let a day or two to stablise. Manipulating ph with such agents can have quite some effects if an incorrect swing happens so better to proceed bit by bit.
Actually I think I can give you a better advice. Keep some water from the next water change. Ideally a known quantity and not such a small one. Perhaps 20 litres, measure the ph and then add a tablespoon of baking soda. Either use a power head or else agitate well the water for quite some time. Leave it for a day and then agitate again and re-measure the PH. This way you will achieve the right dosage without risking to harm any of your fish.