Algae control in brackish tanks is a bit different from SW tanks. Still, you need to control the nutrients going in verses the nutrients coming out. This is part of what I'd post for a SW tank edited for a brackish water tank. This also assumes that this is not a planted tank -
Here are possible sources of nitrates and phosphates -
Feeding, especially flake food and not rinsing frozen foods before feeding. This is perhaps the most common issue.
Test your tap water for nitrates and phosphates. If you get any reading at all, use RO/DI water and fortify it to get the brackish water conditions you want. If you don't get readings, you can use your tap water.
"Dirt traps" and "nitrate factories" in the system. In addition, it often pays to vacuum the sandbed. That can trap a lot of stuff, especially if your using undergravel filters.
Low quality carbon can leach nutrients. This is especially true for carbon cartridges sold for hang on the tank filters. If possible use a carbon of the same grade as you would for a reef.
Livestock load on the system. This doesn't seem to be an issue for your tank.
Here are possible ways to remove nitrates and phosphates -
Water changes. Change 1/2 the water and you reduce the nutrients by 1/2.
The use of Skimming, Nitrate and phosphate removal products, Deep sand beds, and Refugiums are of limited value.
Algae Scrubbers do work for some types of algae problems.
Lighting is often a major factor. One advantage you have is that since your not keeping corals, you don't need bright light. You can cut this way back to limit algae growth, and it's about the easiest thing to do. Many FW and brackish water systems use way too much light.
Keep the filters clean, but don't destroy the bacteria base in the filter, if your filter has some sort of bio-media. Just rinse that off in water removed when you do a partial water change.
Manual remove, which you have been doing helps a lot too.
