Boric Acid?

[Dr. Spike]

It is often used as a pesticide.

From the EPA web site: it can be used as for algae control in swimming pools.

It will lower your pH (it is acid after all). EDIT: I am not saying that you should use it to lower your pH. I am just saying that it will do that.

It is an ingredient in some types of antibacterial ear drops and there are Boric Acid eyewashes you can get at the pharmacy. (DISCLAIMER: Do not try to make either of these yourself!)

There are many uses for Boric Acid.

Not sure what the former owners used it for. Maybe pH with that amount?

I am sure someone else can answer this better than I can. After all, I am a newbie as a pool owner.