What type of algae? Different fish only eat particular type of algae, even famous ones like otocinclus....So you might be getting both algae eaters and rays in trouble in vain...It is difficult feeding a herbivore fish if it's not your focus...slow death for the fish..
The biggest trigger for algae is always raised ammonia levels, those you don't necessarily detect on ad-hock basis testing but enough and chronic enough for the algae to thrive. Secondary to that is light, decrease intensity/duration. You have rays, which is a high bioload, ammonia producing fish....Look into upgrading, more water changes, sumps with K1 media, etc......