Straight outta Pool School
Dry acid is sold by pool stores and the pool departments of some big box stores. Dry acid is sold under various names, including PH Down, PH Reducer, PH Decreaser, Lo-n-Slo, etc. Dry acid is best added by pre-dissolving it in a bucket of water and then pouring that slowly in front of a return.
What you don't want is a pile of acidic powder settling in your metallic heater etching the metal and creating leaks.