Nearly all macroalgae comes from shallow water and receive immense amounts of light rarely achieved in an aquarium. You'll want to give them as much light as you can, especially if you want seagrass. If your tank is a standard 35g, 36"x12"x18", then lighting it with LEDs is going to be around $330 at the minimum (Reefbreeders Photon 24). A single, typical Chinese LED panel is around 12" long and will not properly light a 36" tank, or else you could get that down to around $130-150.
A pair of Tunze 6020 (one of their newest) would be easily hid behind rockwork and provide excellent flow, cost is around $43 each.
Unfortunately $330 for lights alone is definitely out of my budget. If I could find a cheaper LED fixture that had white, blue, and red LEDs would that be at least a step in the right direction to you?