Help! Is my Guppy Sick?

So I have 4 guppies in a planted 5 gallon tank and 3 are doing amazing, super healthy, no torn fins, and the most aggressive thing they are doing to each other and chasing each other, but no nipping. The other one though, got his fins a little nipped but they are growing back. He is usually sitting in one spot, not very active. But here's where I am stumped: he has no clamped fins, his poop looks healthy, he is not floating or on his side, eyes are clear and no external parasites like ich that I can see. He is just not as active. He has also lost his appetite a little but is still eating a bit.

Water parameters:
Ammonia-0
Nitrite-0
Nitrate-5

I also have some cherry shrimp and a mystery snail and they are doing great as well as the other guppies so I don't think it's anything in the water...any thoughts?

I don't have an exact answer for you, but waiting for help and hearing nothing sucks.
First off, your guy is gorgeous. Secondly, you said he's eating, but is he noticeably thinner? Has he been sitting at the bottom of your tank or has he developed any curve to his spine? I'm asking to rule out neon degenerative disease, which guppies can get. Have you introduced ANYTHING new to your tank?
I have a 50gal micro-fish tank, planted, with 12 species. My little Embers and my rasboras' get bored with my tank and will go hide or sit in one spot for days and ignore me, which is odd. It took me months to figure it out, but they were bored with the set up. I'm saying this because your guy, if healthy, may just be depressed/bored. I move plants and decorations about every other month and there's a noticeable difference within 12 hours. All fish are more active. They swim more, eat more and are all over the tank instead of in their favorite spot.
If he continues being dumpy, I would suggest a few days treatment with Artemis Microbe-Lift. It's once-a-day, herbal and it will give your guy a little help with his immune system. You should notice an uptick in his mood after a dose or two, and you can use it for just 2-3 days for a boost. I hope this helps.

ExotixMom said:

First off, your guy is gorgeous. Secondly, you said he's eating, but is he noticeably thinner?

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He doesn't look thinner than the other ones, some of them are really bulky but the other guppy I got at the same place is about the same weight.

ExotixMom said:

Has he been sitting at the bottom of your tank or has he developed any curve to his spine? I'm asking to rule out neon degenerative disease, which guppies can get.

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He has been mainly at the bottom of the tank but sometimes will go towards the top and swim around, especially when I'm doing something in the tank, like feeding, testing the water etc. His spine is as straight as the other guppy's. No curve.

ExotixMom said:

Have you introduced ANYTHING new to your tank?

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No, I got him and the other guppies + some plant all on July 21st, all from my very trusted LFS.

ExotixMom said:

I have a 50gal micro-fish tank, planted, with 12 species. My little Embers and my rasboras' get bored with my tank and will go hide or sit in one spot for days and ignore me, which is odd. It took me months to figure it out, but they were bored with the set up. I'm saying this because your guy, if healthy, may just be depressed/bored. I move plants and decorations about every other month and there's a noticeable difference within 12 hours. All fish are more active. They swim more, eat more and are all over the tank instead of in their favorite spot.
If he continues being dumpy, I would suggest a few days treatment with Artemis Microbe-Lift. It's once-a-day, herbal and it will give your guy a little help with his immune system. You should notice an uptick in his mood after a dose or two, and you can use it for just 2-3 days for a boost. I hope this helps.

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I definitely would think this would be the cause but I've only had them for 8 days. I will attach a picture of the tank but there are a lot of hiding places, grasses to explore, wood with crannies, and a lot of places. It's only a 5 gallon but it seems bigger since I added a ton of plants. Then again, maybe he just explored the tank a ton and is bored again.

I will look for the Microbe lift but I heard garlic can help boost his immune system? I can soak the flakes in garlic water.

Thank you!

He could have worms, gill flukes or an intestinal problem, but most likely he is being picked on by the others. Add floating plants like Water Sprite to provide more cover or move the fish out.

I will get some long floater plants. The other fish, shrimp, or snails are completely healthy and not showing any of these signs so he is probably being picked on, like you said. thank you for the help!

I checked on the guppies today and the sick one (Ember) had died. The shrimp and snails were eating him so I took him out and buried Ember in one of my houseplants. The other 3 are doing great so hopefully they don't bully each other.

Sorry to hear he died...