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!!!

I agree wirh dr. N. Mohan..

Make sure you also use clean glassware and not mix it for culturing different species. 

When sub-culturing, work under a hood previously cleaned with ethanol

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