Nitrates that high are toxic. (Nitrates are toxic, just less toxic than ammonia/nitrite.)
I’d do a 50% water change, then tomorrow another half water change (the two combined will change 75% of the water), then this weekend change a third like usual.
(I’d not be concerned with the magnesium since this isn’t molting issues/they’ve been fine with your hardness all along.)
> Ever since I found a thrive root capsule that has floated and leaked into the water they have been dying.
These have copper and other metals in them that are toxic to invertebrates even in very small amounts. Hopefully you already removed it, but keep doing water changes.
Ducksonquack92 
Nitrates that high are toxic. (Nitrates are toxic, just less toxic than ammonia/nitrite.)
I’d do a 50% water change, then tomorrow another half water change (the two combined will change 75% of the water), then this weekend change a third like usual.
(I’d not be concerned with the magnesium since this isn’t molting issues/they’ve been fine with your hardness all along.)
GH and nitrate are high. Keep doing water changes until they come down. Might use distilled water to help lower the hardness.
CO2 can make the water acidic too, have you upped the dosage or anything?
> Ever since I found a thrive root capsule that has floated and leaked into the water they have been dying.
These have copper and other metals in them that are toxic to invertebrates even in very small amounts. Hopefully you already removed it, but keep doing water changes.
I quickly browsed the comments and I don’t see anyone has said to check your copper levels. It’s toxic to shrimp
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U should have enough plants to keep nitrates very low. Or do water changes to lower them.
Do a 50% water change and then learn to use Google. JFC, the answer is right there on the API test kit.
Put more live plants to get those levels under control.