You have to know what you are doing. The addition of muriatic acid lowers the pH and total alkalinity in Swimming Pools.
The water becomes aggressive and can etch the plaster and metal parts of the pool and equipment.
I have treated for metals and algae and now have a problem getting the chlorine levels up. The ph, hardner and alkilinity is ok.
take out the phosphates!
Rain if it is acidic can lower the pH of a swimming pool.
You use muriatic acid.
By adding muriatic acid
a swimming pool a swimming pool
Buy some PH down.
add a PH reducer chemical