Angelfish pair up. Look up spawning behavior and stuff and if you see them exhibiting it there you go. One day you might wake up to find a bunch of eggs lain on you filter intake (they like to use it sometimes if there isn't any flat rock around). Of that happens you can move the eggs to a tank where other fish won't eat them.
I say if you think you have a pair and they can fit comfortably in your extra tank (29g or larger) then move them there to live. After they spawn a couple times they learn how to take care of their eggs and will raise the babies (some aren't good parents some are) you would still need another tank to grow out all the babies I think though unless they lay on a flat rock and you move the babies to your extra tank to raise them yourself.
Im not the definitive expert bit this is some of what I've heard when asking this same qiestion