[Jensen]

For lettuce, I think tap water is fine. Never used lemon juice before. One suggestion would be to keep in a cool dark place for the first night after planting and watering the rock wool. For example commercial farmers have an air conditioned room which they leave their newly seeded rock wool. The cool moist air helps break any dormancy the seed may have. A low dose of hydroponic fertilizer might also help. Maybe 1/4 strength of the regular hydroponic solution.