Selectmen have set a special meeting for 7 p.m. Feb. 19 at Town Hall to discuss water-quality issues at the Monponsett ponds. Officials will answer questions on the plan to treat West Monponsett with aluminum sulfate this spring to help control algae growth. Town Administrator Charles Seelig said the meeting will also focus on Brockton’s water diversion from the Monponsett ponds as part of the city’s water supply, and whether Brockton should be doing more to help address water-quality issues. Halifax officials have voiced concern for several years over lack of adequate downstream flow from the ponds. Brockton Water Systems Manager Brian Creedon will be at the meeting to answer questions, Seelig said.