Try a different plant perhaps? I used to kill all houseplants, even the ones labeled hard to kill, until I got into orchids and they flourished for me. So I think not all plants work for all people, and even more so not all plants need the same environment to flourish. Perhaps your tank is currently not good for swords. You could address this by trying to adjust your tank environment to work for swords, but another approach would be to try different aquatic plants to see whether other plants will work better in your environment.
My understanding is that there is no single test kit for plant nutrients. The API test kit does test nitrates, which are one plant nutrient. There is a separate test kit for phosphates. I believe you can also buy one for Iron for example. And there may be others.
I have two 54W T5HOs on my 55 gallon, and my plants are doing great. It's all about a balance between light, ferts, and CO2. I only have the lights on for 5 hours daily, how long are yours on? They're very bright, so too much longer and you'll have algae problems. I also dose dry ferts and have injected CO2. For larger tanks, dry ferts are much more cost-effective than liquid ferts.
There's nothing wrong with liquid CO2, I used it for awhile. It's not as effective because the plants have to break it down to access the carbon, but the reason I went with an injection system (which was only about $220 pieced together, rather than buying a kit) was because if you spend $12ish bucks on a bottle, eventually you'll end up spending the amount it would have cost to buy an injection system
I dunno what it is about swords, I can't seem to grow them. My tank has everything a plant could desire, but my amazon sword isn't really growing, and some of the leaves are brown on the tips
I just started the flourish, so I'm hoping it can make a comeback. Dealt with black beard , diatoms, and low light and am hitting the ferts now. I'm about three days in. I wanna say its better but I won't until I see all is good