When I use 20 mule borax and acid, what else is produced.

Have you considered granular boric acid instead? It's much easier to add than borax & MA.

Borax dissolves as boric acid into water:

Na₂B₄O₇ • 10H₂O --> 2Na⁺ + 4B(OH)₃ + 2OH^- + 3H₂O

The 2OH^- are what raise pH in the process.

Boric acid is then in a pH-dependant equilibrium with borate ion:

B(OH)₃ + H₂O <--> B(OH)₄^- + H⁺

Adding muriatic acid will simply decrease pH, moving above equilibrium a little bit to the left, basically turning some borate ion into boric acid. But at pool pH, boric acid is the dominant species anyway:



There is no by-product from the neutralisation. The hydrogen ion from the muriatic acid addition simply neutralises the hydroxyl ion from the Borax addition, decreasing pH which makes all chemical equilibriums rejig a bit, not just above equilibrium between boric acid and borate ion (carbonates, CYA, chlorine, whatever is in the water)

You could probably say that the by-product from adding borates via Borax and muriatic acid instead of straight boric acid is salt.

Borax adds sodium ions, and muriatic acid adds chloride, that you both wouldn't add by adding plain boric acid.