Just a follow-up, spongepower (my bottle from 2020) doesn't add detectable Fe or interfere with the Fe test, even at 100X recommended dose.
I used the hanna low range Fe checker to measure tank water, and added a dose of Red sea Trace C "Iron+" that should result in 100 ppb Fe. This data is in Blue below and shows the expected result. Undetectable in tank water, and near ~100ppb after addition of the red sea Fe product.
Then I did the same thing but with 1 drop / Liter of SpongePower (100x dose), shown in Red.
It neither added Fe nor interfered with the detected Fe from the Red Sea Iron+ product.
my bottle is from 2020 like I said. Since they list no ingredients, I suppose that makes it easy to change them any time you want