What causes red tide?

Red tide is caused by microscopic algae that produce toxins that kill fish and make shellfish dangerous to eat. The toxins may also make the surrounding air difficult to breathe.

You mentioned microscopic, is it about very small algaes and not about the big ones that causes red tide?

Yes as it is because of higher-than-normal concentration of a microscopic alga plant-like organism. In marine saltwater environments along Florida’s west coast and elsewhere in the Gulf of Mexico, the species that causes red tides is Karenia brevis.