We recommend water quality testing once a year for tap water, and twice a year for well water.
Literal
Tap-water [with-で] year 1-time, well-water [conditional-なら] year 2-times water-quality-testing [object-を] recommend.
A practical advisory. ~なら provides a conditional fork: 'if it's well water, then twice.' 年1回 and 年2回 use the counter 回 for frequency. お勧めします is the polite recommendation form. The sentence efficiently handles two cases in parallel.