It's not big complicated work my suggestion is you have to follow GLP very strictly. once you isolated the monoalgal culture thereafter you have to periodically subculture it once in 15 days or based on its growth. The way of handling is the magic here!!!
Check on your water source, autoclave all apparatus including the water media and check on your air supply. I depend on portable aerator to minimise contamination from using air supply from our hatchery