Yes, baking soda can kill algae in ponds if you put enough of it in there, but that’s the problem. You can’t put enough of it in there.
The maximum amount of baking soda you can safely apply to your pond is 1 cup per 1,000 gallons of water, and that’s only if your carbonate hardness (KH) is already too low. If it’s not too low, you won’t need to add any baking soda at all.
And anyway, most people don’t have a pond that’s over 1,000 gallons in size, so you’ll need to adjust the amount of baking soda you use accordingly.
If you add more than 1 cup per 1,000 gallons, you’re in dangerous territory. In fact, I’d say that half of that is the real maximum.
You see, KH acts as a pH buffer. It prevents your pH from going all crazy, and more importantly, it keeps it from falling too low.
If you add too much baking soda, your KH will rise too high and your pH will become too high as a result. It might even start fluctuating wildly.
Believe me, a high and/or fluctuating pH is far worse for your fish than a low pH. Fish can handle pH levels all the way down to 6, but anything outside the safe range of 6.5 to 8.5 can be very stressful for them.
A high KH is also not a good thing. It can harm your plants, and it’ll make it difficult for you to adjust the pH of your pond, because the high KH will just keep bringing it back up again.
The Bottom Line
Baking soda is a very imprecise way to kill algae in your pond, and it’s far too dangerous to be worth the risk.
It’s much better to use an algaecide that’s made for ponds, because they’re proven to be safe for your fish and plants when used according to the instructions.
If you’re still not convinced, you might want to look into barley straw instead. It’s a completely natural way to kill algae, and it’s much more effective than baking soda.
Just make sure that you’re getting the right kind of barley straw. There are lots of barley straw products out there that are just made to be a cheap alternative to straw bales for gardening.
Those products will not work to kill algae, so make sure that you’re getting genuine barley straw that has been treated with the right amount of UV for the right amount of time.
