I dont think you have much to worry about there, as long as the tablets stay buried.
I used to dose iodine under recommendation years ago but stopped with no ill effect. I believe proper diet gives them what they need. Not sure what the common thought today is among shrimpkeepers regarding to iodine.
I don't think fert tabs would cause any harm unless you stir up the substrate to much and release a bunch of the nutrients. Even then I don't know exactly what would happen. You also don't need to dose iodine for your shrimp.
Trouble with fert tabs is - the roots form a clump around it and when a plant is pulled out of the substrate, the fert tab easily dissolve as the plant is taken out of water. Doing so, releases a lot of nutrients - and more, if you happen to pull several stems/roots with fert tabs.