Involved in phosphonate degradation. H2O + phosphonoacetaldehyde = acetaldehyde + H(+) + phosphate Binds 1 Mg(2+) ion per subunit. Homodimer. Induced when inorganic phosphate is limiting; this is controlled by PhoB. Maps to a phosphate-starvation-inducible locus previously known as psiC. Belongs to the HAD-like hydrolase superfamily. PhnX family.