That Cal Hypo or shock just about kills everything. That and muriatic acid I doubt any plant could survive. If you want hyacinths to survive I would think you are going to have to refrain from that stuff and hopefully let it dissipate away.
I see your point on the cages I would hope kids and people would just leave them alone. Probably wishful thinking on my part. I would think some kind of fertilizer runoff is knocking your PH out but could be many things, even organic. Do you have sediment on the bottom of this pond?
