Most of my tomato plants are doing really well. But the one in the cage highlighted in red started failing a few weeks ago. I fertilized with some Miracle grow I had lying around and it didn’t improve. Then I got some Tomato Tone. That did nothing. The plant kept getting worse and worse and now it’s just a husk. I’m afraid blue is following next and I’m concerned about purple. Here’s another angle for some more context. 
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Any thoughts?
I water almost every day. How do I know if it’s too much?
Every day is probably too much, unless you’re in a dry area and they’re very well drained. I’d describe watering as saturating, not flooding, and let it dry to the roots, not just the surface, before watering again. Dry, but not completely moistureless bone dry
Also, on hot sunny days, water in the evening or early morning, else you’ll just lose a lot of water to sun evaporation.
When it shows signs of over watering, they only need about an inch a week after being established. When they are new or transplanted water daily for a week, than give it an inch of water a week. If it rains an inch, you’re done for week unless it’s stupid dry and starts showing signs of needing water (obviously).
You could always get a moisture probe. I’ve found it extremely useful
I doubt it’s over watering, due to the fact the others look healthy enough. Dig up the root ball of that one, look for pests in there would be my first step to diagnosing.