My pond is running about pH 8.0. Which I think is marginally high - what should I add?

From my tropical aquarium keeping days, I remember that there are household products safe for use in changing water pH. I know baking soda was one of them BUT, I don't recall any of the others.

Does anyone know what they are and which does what ?

A pond can live at pH 8.0 although it is a little high. If it doesn't go any higher, I probably wouldn't add anything. Baking soda (sodium bicarbonate) will raise the pH. Most commercial pH down solutions contain sulfuric acid. Being a chemist, I can get chemicals that most ponders can't. I would probably use hydrochloric acid which you might be able to get as muriatic acid at a pool store but you have to be sure it's a pure acid without other chemicals. All these have risks because you have to add a dilute mix of it very slowly over a long time and monitor it carefully. If you dump too much acid in one spot, you'll kill animals and plants. That's why it's safer to use the commercial stuff or not bother. As for household acids, vinegar (acetic acid) can lower the pH a little but it's not strong enough to be worth using in big ponds. It takes a lot of tinkering to get it right so it's usually not worth it. Also, find out if anything in the pond like cement or limestone could be raising your pH (and compare to the pH of the water from the tap).