chaeto slowdown shrinking

Over the past 10 days my Chaeto is shrinking in volume . I had a nice growth that was fluffy light and distributed by volume in my entire sump area it belonged , its now shrinking and heavy and is almost 25 % of the space it occupied. It is not dying. What could be the reason ? My Ph is stable...

Over the past 10 days my Chaeto is shrinking in volume . I had a nice growth that was fluffy light and distributed by sixe , its now shrinking and heavy . It is not dying. What could be the reason ? My Ph is stable 8.2 to 8.1 My Ammonia is constant .007 Nitates 0 Alk 8 Calcium 413 Mag 1299...