How best to clean red brick in a patio. Had 30 years of standing water

Be careful because the bricks will be very soft and may crumble if you try to scrub them etc. Might be better to consider a Brick & Tile Paint,

TSP (trisodium phosphate).

Solve the standing water problem first then clean it, otherwise you'll have to clean it frequently.

Use Wet and Forget a safe cleaner for all surfaces

TSP first, then rinse (i WOULD USE PRESSURE WASHER, BUT NOT TOO HIGH) then another water rinse with some bleach to prevent/ help kill any mold spores, and improve drainage for future.

I assume you want to use the patio again? First the water problem has got to go away, permanently. Let the bricks dry out for several weeks and determine if there is any damage or what has settled onto them. Then, clean. All the suggestions above are good, depending on the tools you have and how energetic you are. You also may need to seal them once your done. But what they look like at that point will determine this.

Porus Brick absorbs water, Bacteria, etc. colors brick, requires Dye, Bleaching, etc., or Grinding, far as I know. Altogether Too Many Poisons in our Environment already. Choose Wisely. Good Health, Everything Else is Over-rated!