You know, I had a difficult time with chaeto in a first couple of months - it became dark and disintegrated into a small pieces, floating in the tank - really annoying. Then the top portion started to grow.
No relation to light (tried sunlight, 6,500K, 4100K, now have one portion under Melev's flood light, but soft white, ~2,300K, will see), may be 4,100K 13W was not so good as 27W 6,500K

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No relation to pH, alkalinity (I have more than one nano-tank), when nitrates above 20 ppm, phosphates 0.5 ppm - grows faster, than with nitrates 80 ppm and phosphates 1 ppm (had this for some time, could be other unaccounted factors too).
Give it a time, or light, or phosphates...
One more thing - the ball of chaeto grows better for me, when it's loosely packed. In deep tank or bucket the bottom portions started disintegrate again. Reduced light? But I tried adding side light too. Who knows.