Question

What organisms are most often involved in red tides? In my own words

Answer 1

Short version you can use:
"Red tides are blooms of tiny, single-celled algae—mostly dinoflagellates—that build up in seawater; when they get very dense they can turn the water reddish and often release toxins that hurt fish, shellfish, and people."

If you want a bit more detail in your own words:
"Red tides are caused mainly by microscopic marine algae called dinoflagellates (examples include Karenia and Alexandrium). Under conditions like warm, calm water and excess nutrients, these algae multiply into dense blooms that can discolor the sea and produce toxins or reduce oxygen, killing fish and contaminating shellfish. (In freshwater, similar blooms are often from cyanobacteria or certain diatoms.)"

Pick whichever fits the tone you need.