bomber,
If you're getting algae, I think we can conclude that your numbers are not fine. Insufficient chlorine for your level of CYA is by far the most likely reason for the problem. While poor filtration might exacerbate an algae problem, I think it unlikely that that's the root cause. Can you post a full set of numbers?

On your sand filter, what size is it? Does it have sufficient sand in it? Too much? Does it have filter sand and not play sand in it? A properly sized and properly functioning sand filter won't have the problems you're having.

Cheers,
Kurt