I can't see your pictures, but regardless, your next step is to do some testing. Get some gypsum (I bought a $4 bag of pelleted gypsum from Menards) and stir known amounts into a 5 gallon bucket of pond water and see what happens. Gypsum did not work for me.

You can also try the vinegar test and lime tests too if you wish to avoid alum. Although you might as well try the alum too just to see what it does.

[Linked Image]