[somewhatshocked]

It was cycled using seachem stability for 1 week.

That, unfortunately, did not cycle your tank.

Read this about the fishless cycle. That's how to cycle a tank. It'll be helpful for you to keep the link handy. You'll need to feed your tank an ammonia source on a daily basis for about a month on average before livestock like shrimp can be moved in.

After the 2nd week, I tested the water and nitrite/nitrates were all at 0 so I added 50 cherry shrimps.

Adding all of those shrimp (plural) to an uncycled tank means they're all going to die unless they're removed, unfortunately. It's going to take several weeks for your tank to be ready for them at minimum.

Is it the fertilizer? Or is it the seachem stability that didn't work. Or is it the dead 20 stems or so of jungle val or something else?

It's that the tank is just now beginning to cycle.

I’m assuming the tank is just now starting to go through its cycle.

Yep. This is what's occurring.

At this point what do you suggest I do besides dosing prime/stability and water changes.

Remove all of your shrimp and keep them in a separate container. They can't survive ammonia, nitrite and the like. Keep them in a bucket or something, dose it with Prime daily and do daily water changes until your tank is ready for them. Leaving them in the tank means certain death.

But you can do it. It won't be a daunting task. Just take it step by step.

 

[somewhatshocked]

I just remembered something.

I brought/installed ammonia and nitrite filtering pads for the fx4 last week. Is that beneficial or bad?

Not helpful in this case because your tank doesn't have much in terms of the nitrogen cycle going on yet.

Read this. For real. It'll be supremely helpful. It's the link I shared earlier with a quick bit that easily explains the fishless cycle.

Move all of your livestock from the tank. The shrimp are going to die if you don't. No amount of filter media will change that and unfortunately, water changes and Prime alone won't save all the shrimp. You're going to need to get all the shrimp into a separate container until the tank is safe for them.