Dwarf sag turning yellow.

My dwarf sag is turning yellow and melting.
I planted it in quikrete pfs with flourish root tabs.

I think it could be low iron, but should I add more tabs?

To my knowledge, they will not use liquid ferts. Is that correct?

Any other ideas of what the problem could be?

This plant, Sagittaria subulata or commonly dwarf Sagittaria, for some reason does not grow in some tanks. You could use a comprehensive liquid fertilizer along with the substrate tabs. Flourish Comprehensive Supplement for the Planted Aquarium is good, and works with the Flourish Tabs. Do not dose iron in addition or you could kill plants, fish and bacteria; there is proportional iron on both products sufficient for any planted tank that is low-tech or natural as here.

Kasselmann (2003) writes that this plant does best in moderately hard to hard water. I had this plant for a time, but it suddenly died out after several months. My very similar Helanthium tenellum, the pygmy chain sword, thrived for years. If you can find one of these, give it a try. Once established, it will send out runners with adventitious plants by the dozens.

If it's new, it could possibly be that it was raised emersed and transitioning into submersed

Thanks for the help guys. I'll check out that plant @Byron. Thanks for the suggestion

Here's some pictures of the plants. They are all suffering.

Agree, I would start using the Flourish Comprehensive Supplement (liquid). Substrate tabs are of very limited value to stem plants, but they do greatly improve plants like swords.