New aquarium owner here! Is there something wrong with my guppy?

Hello! My guppy, Remy, has been lethargic and I've seen him resting on the bottom of the fish tank for the past few days. He eats and will move if provoked. I'm unsure if he's sick or if 'he' is actually a 'she' and is pregnant! I've had Remy for a few months, and up until now he was an active and smart fish. A week ago I introduced a guppy into the tank. Is Remy stressed out from this new fish?
The other fish seem fine and are swimming at all levels of the tank.
Basically, is my guppy acting normally, or is he stressed/diseased?
Current setup is 2 neon tetras with 2 guppies, 5 gallon tank

 

What is your water parameters?

 

Hello! My guppy, Remy, has been lethargic and I've seen him resting on the bottom of the fish tank for the past few days. He eats and will move if provoked. I'm unsure if he's sick or if 'he' is actually a 'she' and is pregnant! I've had Remy for a few months, and up until now he was an active and smart fish. A week ago I introduced a guppy into the tank. Is Remy stressed out from this new fish?
The other fish seem fine and are swimming at all levels of the tank.
Basically, is my guppy acting normally, or is he stressed/diseased?
Current setup is 2 neon tetras with 2 guppies, 5 gallon tank

Is he going to the top for air?
Is the water temperature in the 60s, if you don't have thermostat, what's the room temperature? The cold makes them slow down.

You'll get weird reactions the first few months, you have to wait for the biosphere to develop. Orrrr short cut;

One of the most valuable things you should do that will eliminate most your problems is to buy a live low light plant from your pet store that is full algae and all kinds of benefitial stuff, even just a piece wood or any decorations that they would sell you from their established tank. You have to carefully bring it home submerged without disturbing the live that grows on it. You should get this asap.

Be on the look out for white bubbles on the fish, that would be ick. This starves their oxygen which is one symptom and they start to slow down.

 

I am 99% sure that you definitely have a male, so the pregnancy thing is out. I know this is a really beginner question but is the tank cycled? Also have you checked the ammonia? His gills look a tad red to me, that can mean ammonia poisoning but you'd have to test the water to know for sure. Lastly did you remember to use decholinator? Sorry for all of the dumb questions.

 

Thank you for replies!
Yes, the tank has been cycled. It's been running for about 3 months now. I have 5 live plants, a piece of driftwood, decorations, a heater, air pump, and filter. I changed the water a couple of days ago, and I use API tap water conditioner.
I'm glad to hear pregnancy isn't going to happen; I was not prepared for it! Remy does not seem to be gasping, but you're right that his gills are a bit red.
pH: 7.5ish
Nitrite: 0 ppm
Nitrate: 20 ppm
I do not have the materials to test for ammonia, but do these stats give enough insight?

 

The guppy photo is definitely a male.

Not having an ammonia reading suggests you're using test strips. Test strips are notorious for being unreliable. Most folks on this forum us the API Master Test Kit with test tubes and indicator solution. It will test for pH, ammonia, nitrites, and nitrates.

How often do you do partial water changes?

If you added another fish a week ago, there's a good chance that fish brought something with it that's not good.

Your guppy is not acting normally. If no other symptoms appear soon with that fish or the others, it's possible it's being picked on. Keep an eye open for other signs of discomfort. If it's ich, tiny white spots will appear and look like someone sprinkled table salt on your fish.