They are stressed. In the second picture, I see some brown mass, it appears under stress also. Since you kept a light regime of 24/24 h illumination you are stressing the culture. Also, the temperature is a little higher than optimal. I recommend you keep your stock cultures at 25 °C and photoperiods of 16/8 h (8 h dark); unless you are trying to do something else.

Froylan M.E. Escalante Thanks for your reply. I do 24/24 lighting because I want to propagate quickly to transfer to open pond.

In the research I read it seems that spirulina grows best at 32-35 degrees Celsius, so I maintain that temperature for the environment

Continuous aeration, especially if it's too vigorous, can cause mechanical damage to the algae, leading to fiber breakage and clumping.

While Spirulina benefits from continuous light, excessive intensity or improper light distribution can stress the cells, causing them to clump and break.

A sudden change or imbalance in nutrients could weaken the algae, making them more susceptible to breaking.

Although the pH range you maintain is generally suitable for Spirulina, slight fluctuations or sudden changes might stress the algae, leading to clumping and broken fibers.

The presence of contaminants (like other microorganisms or debris) could cause physical or chemical stress, leading to clumping and fiber breakage.

Although Spirulina can tolerate temperatures up to 36°C, sustained high temperatures close to this upper limit might cause stress and damage to the cells.

You might want to check the aeration intensity, light intensity, and nutrient levels, as well as ensure the culture is free from contamination.