Thanks.
Ella

I'd stick to the baking soda so I know what I've got and it's easier to judge how much you need by getting it in the box already weighed rather than guessing how much water is figured into the other stuff and how much of that it will take.
Also, I bet baking soda is cheaper.

Washing soda is sodium carbonate and has a PH of around 10. Sodium bicarbonate has a PH or around 8. You can make sodium carbonate (washing soda) from sodium bicarb by heating it. It releases carbon dioxide and steam in the process.

As to why you would want to bring up the pH with baking soda, it can be beneficial for a number of reasons. Organic processes tend to acidify the water, so adding an alkaline buffer is often a good idea. Ammonia is an interesting situation. It is more toxic at higher pH, but will gas out much more readily. Overall, lower pH promotes the accumulation of ammonia, nitrite, and nitrate, which is something we generally want to avoid in a pond with fish.

This will of course depend on the situation. Some tropical fish are very sodium-sensitive, but they also tend to like a lower pH, so baking soda would be undesirable. Goldfish and koi, on the other hand, prefer a slightly basic pH and are quite sodium-tolerant. In fact they can thrive in slightly to moderately brackish water. You always have to be careful and know what you're doing. There is a lot of good info in this forum.

My prefered method is to add lime, dolomitic lime, and calcium carbonate materials like shells or coral to the pond for long-term buffering, and add a little baking soda (~ 1 tsp/ 5 gallons) to new water when I'm doing changes. This way I'm in no danger of adding too much sodium, since I also have pond plants and use the old water in the vegetable garden.

Thanks for the information. Adding the wash water to a pond may not be a good idea, maybe have to wait only until it sits a while, maybe with some plants in it. All input welcome here, this is all new to me.
Thanks,
Ella
who hopes th e computer really is working.

You'll certainly want a way to measure pH before putting living things in the pond, so this could be a good opportunity. Add a little of the wash water, see what it does to pH, add a little more, etc. You can also experiment with neutralizing it with something acidic. Gypsum (calcium sulfate) comes to mind.

But without knowing how much washing soda you start with and the volume of your pond, it's hard to say whether you'll be adding too much sodium. pH can be balanced and buffered, carbonate leaves the system through photosynthesis, CO2 outgassing and lime precipitation (along with calcium), but sodium can only be removed by changing water.