BNs are fabulous algae eaters. If you have some large, aquarium-safe smooth stones, put them in some dirty tank water, set on a window ledge, and let the algae flourish, then place it in the tank.
You can feed them a wide variety of veggies. Lettuce is good if it's dark green (dark green veggies are the best for everyone!), but you can also feed them spinach, cucumber, zuchinni, peas, carrots, basically any veggie or fruit. Mine even nibbled on a banana some, but the swordtails decided they liked it far more.
I wash a fork REALLY well, wipe with an antibacterial bleach wipe, rinserinserinserinserinse, wipe, rinse x5 again, then place in a container with dechlorinted water while transporting it to the tank. Either stab the veggie or just put the fork on top. You can also tie it to a stone with fishing line or rubber bands.
One thing to remember is that BNs are nocturnal. Drop an algae wafer in after you turn the light out every couple of days. Your BN will sniff it out pretty quickly. Mine pounces on his algae wafers now, swats away anyone who tries to take some (which is basically everyone in the tank!), and then drags them back into his daytime "cave"...it's hilarious to watch and the reason he quickly got named Howard (as in Hughes
For a great article on feeding veggies to fish: