chaeto grows best in light and a pretty decent amount of flow in my opinion. If there is no flow through it, you can actually get cyano growing on and in the mess of algae lol. It also seems to grow faster with some decent flow. I guess the flow is like anything else it helps push the nutrients to the chaeto and carry away the cleaner water. I'm not talking about a huge powerhead pushing on it but something to push a little water across it will help it I use a nano or a number #1 koralia. Gracilaria is also a great algae to grow in a fuge. It's nice because fish eat it and it grows at a good clip. it does need a little stronger flow than chaeto though and should be tumbling gently in the fuge rather than just sitting.