Anjampie

What makes you think your plants are having trouble using nitrates? You could try the Estimative Index fertilizer method using "dry salts". That will save you a pile of money over liquid ferts and it only requires one weekly 50% water change, so not much different from your current water change strategy. I've been using the EI method for a while now am I'm happy with it: https://niade.com/estimative-index/ but others have been using this for many years very successfully - the idea is to have the various fertilizer components present in balanced levels that mean plant growth is not limited. It sounds intimidating but it's really not once you give it a go.

I don't know it's the prettiest, but currently I'm tracking my Bucephalandra caterina with interest.

Sorry for your experience. Some component has failed, either the regulator or the needle valve and actually I might suspect the needle valve. Can you post which type of needle valve you're using - is that also from Ista? In my experience the regulator plus needle valve combo requires a lot of control from both the regulator and the needle valve and even very small changes in the needle valve setting can make large changes in CO2 flow. You can help make the system more robust and less "twitchy" if you insert a secondary flow restrictor between the regulator and the needle valve. These are very affordable and help bring the flow rate into a more easily controlled range. Here's the set-up I went with for my nano aquarium: https://niade.com/controlling-aquarium-co2/