@Mr.C.Growin4B to prevent algae while you’re gone 1 copper penny per gallon (pre 1982 penny) or a piece of copper pipe weighing roughly 2.5 grams per gallon of water (a copper penny weighs 2.5 grams).
Another thing that works great as a preventative is 1 tsp of apple cider vinegar per gallon. It prevents algae growth, will kill anaerobic pathogens, and it also has some micro nutrients in it but not enough to really effect your nutrient system or feeding schedules. Just double check your ph before you leave and make sure your pump and air stones are in tip top shape to keep ph under control.
Do you have your watering system temperature regulated? Water temp regulators help a lot keeping ph and other issues under control as well especially if you need an automated system that can handle days or weeks at a time without maintenance.