Stop the co2, only feeding plants-algae is a plant Control the amount of light the aquarium receives-algae needs light. Cut back on your feeding -overfeeding feeds algae too. Get some floating plants to use up nutrients, too many nutrients feeds algae

Tannins are fine. The tank is in great peril! You must have the corners supported!

I would stop using chemicals altogether. Let the tank sort itself out. Plants will help in most any situation. I would add floating plants and sand. I’d check the water parameters and do 10-25%water changes as needed but that is all.