[Doug1]

" I do 20 gallon changes about every 3 weeks. The trace elements I add weekly are all Kent Marine Products: Tech M; Strontium & Molybdenum, Coral Vite, Liquid Calcium, Idodine, and Cycle. "

Water change is fine per se, but stop wasting money on all the other stuff.
Have you tested your source water(tap) for nitrates or other contaminents, as a matter of fact are you using RO/DI water, what are the Ammonia, nitrite and nitrate readings, and yes if you have cyanobacteria you have excess organics

 

[Doug1]

A readable trace of ammonia is not good, but that might be shortcoming of test kit
You mention readable phosph, along with a nitrate reading of 20. I am betting the actual levels are higher and the proliferation of cyano is locking a lot of it down.
Cyano are among the oldest5 living organisms and they didnt in that honor by leaving when asked. You are never going to completely rid tank of it nor should you want too, Its actually a vital link in the food chain. Its just when conditions allow to gain a fothold and run rampent as the red slime mats that its problematic. Best bet is to siphon out as much as you can during water changes or use Spankys filter floss in the colender trick if you dont have a sump. Cut back on feeding, and additives. Your pH,and calcium look good, alk might be a tad low, it works out to 8.4 Dkh 10 is median recommended. The Stronium is a debated trace element, Molybnium is another that there is not stadard RDA for, and Iodine is like the chem version of best light. At the least you shouldnt need to add it, ya get enough from food and water changes
Remember it is a POWERFUL ANTISEPTIC and toxic as hell. Again there is no standardize dailey dose or desired target level.
Lastly do you RO/DI water for topping off evap and making up new SW. If not you are probably introducing things that are effecting your system