I'd argue that tank is too small for either fish. Recommended sizes for those fish are 4ft plus. You could get small ones and move them on later.
HOwever, that's addressing the symptoms and not the cause. The cause is most likely high nutrients, even though you're not seeing them in the test results, Algae is consuming it (or it wouldn't be growing) so it's not in the water column to be tested for. Lighting at 6hrs is ok. I'd go with reduced feeding and several large water changes. Also rip out as much as you can by hand. That will reduce the algae's ability to suck up nutrients, reduce the amount of nutrients going in, and reduce anything in the water column that being broken down as waste from feeding, and from dying algae as it rots.