Blocking Light in Sump?

[eternitybc]

Just to clarify,the main tank doesnt have algae, just the refugium. Lights are on 24/7.

Phospates/nitrates/nitrates are always at 0 when tested, and we use phosphate and carbon, which is changed regularly. Fish are fed daily, only what they can eat, frozen brine shrimpw hich is well cleaned. We tried cutting back on feedings, it didn't help. So out of all the causes of algae, it looks like our lighting is the issue (5 t5's mounted to the side of the tank, covering the entire front of the sump)

The algae isn't overwhelming, but having to pull the sump apart every 2-3 months to scrub everything down is getting annoying, and when we reduce the lights to 12 hours our chaeto turns yellow... so preventing the light from going through the refugium into the rest of the sump seems like the best plan.