I live in the tropics, and have a water tank that sits in the sun, forming algae. Every year, I need to scrub the thing out. The plumber I’ve hired for the job wants to flush my water pipes with HCl to kill any residual algae. The pipes are mostly PVC, with braided metal hose and metal fittings here and there. Should I let him? If not, why not?
Are they metal pipes? If you want to kill algae as such, flush with a diluted quaternary ammononium disinfectant, let sit a day, then flush everything very well indeed, and again. If you have rubber or polymer fittings, don't do that, either, for the stuff diffuses in.
Clean the tank with bleach and flush it several times. Once clean, put a piece of copper pipe in the tank and that should help it stay clean. Don't use HCl, too strong for the job.