Use a pure ammonia source,... Go to the store!
Or you could empty your skimmer cup from your other tank into it.
Or you could better yet, buy some micro-algae grow from a dealer. That would have all the nutrients plus the trace to keep it alive and growing.
You could also just get a small school of chromis in the tank with it. Chromis are cheap, (and I have no emotional attachment to them)
You could even ghost feed, but chromis would be better.
My skimmer and phos reactor have almost killed back all my chaeto unfortunately. I think I'm goin to take my reactor offline for a while and see if I can get better algal growth.