You may not have enough dissolved oxygen. That would affect your bacteria growth. If that is the case, then adding another grow bed might help if it is flood and drain. If, however, you add more fish at the same time, then you will be right back where you started.

What is the timing on your flood and drain cycle in your current grow bed? The more often you cycle your grow bed (up to a point), the more DO you will have in your system.

Also, as bacteria wastes increase (be it Nitrites or Nitrates), the bacteria have a harder time reproducing. This is why ample DO is so important.

Oliver