5 Gallon Tank Lone guppy keeps at the bottom of the tank

Hello, I had 3 male guppies in my 5 gallon tank and recently two of them died. I performed a water change and made sure amonia and nitrite levels were in check. Yet my last guppy has gotten very lethargic, keeps at the bottom and only moves when food falls down near him (still has an appetite and would swim up occasionally, but non of that hyper guppy swimming). I can't see any spots or signs of infection on him other than some "red thread" hanging off of him (might be his poop but has been there for a while now).
Maybe someone more experienced could please look at the attached picture and see if they can tell what's wrong with my fish? I'll be thankful for any suggestions.

 

What are your exact parameters? His fins look clamped which is usually a result of stress.

Beside his fins, guppies are social fish. They enjoy the company of other guppies. Once you rule out water quality I suggest getting more guppies to keep him company.

 

What are your exact parameters? His fins look clamped which is usually a result of stress.

Beside his fins, guppies are social fish. They enjoy the company of other guppies. Once you rule out water quality I suggest getting more guppies to keep him company.

Thank you for your reply. I just retested the water and: ph is 7,2 amonia and nitrire are 0 (I don't know how exact my tests are, I am using stuff from Aquar). I keep the teperature at 80F.
I was considering adding more guppies but didn't want to cause them to die too in case there is some disease in my tank. Do you think I should get some medicine and apply it even though I can't see any visible signs of a disease? I could get for example Sera Baktopur where I come from (Czech Republic).
Thank you in advance for your reaction.

 

Thank you for your reply. I just retested the water and: ph is 7,2 amonia and nitrire are 0 (I don't know how exact my tests are, I am using stuff from Aquar). I keep the teperature at 80F.
I was considering adding more guppies but didn't want to cause them to die too in case there is some disease in my tank. Do you think I should get some medicine and apply it even though I can't see any visible signs of a disease? I could get for example Sera Baktopur where I come from (Czech Republic).
Thank you in advance for your reaction.

What about nitrate? How often do you do water changes and how much?

Imo medication should only be used as a last resort and if you know exactly what you are treating because it can be very hard on the fish. I also don't see any visible symptoms of disease. Did the guppies that passed away exhibit any symptoms or change in behavior?

 

Also one more thing, If I get let's say two guppies from one 'batch", won't they bully my current guppy? Sorry if it's a stupid question (aka no way to tell)

What about nitrate? How often do you do water changes and how much?

Imo medication should only be used as a last resort and if you know exactly what you are treating because it can be very hard on the fish. I also don't see any visible symptoms of disease. Did the guppies that passed away exhibit any symptoms or change in behavior?

They didn't. Their fins were still beautifuly colored and no spots or anything on their boddies. One died one day and the second just the day after that.
I don't know if this is relevant, but I found a snail (the size of a hazelnut) in my tank the day the second fish died. I guess it came with one of the plants that I got a few months prior to that. Do you think it could have somehow hurt them or be toxic or something? (I am a total beginner in fishkeeping if that's not obvisou... I was gifted the fish without being asked if I'm up for it. So sorry if what I am asking is riddiculous, I am just trying to figure out what's wrong and how to make it better).
Oh, I don't have a test for nitrates. I am gonna go buy it now and let you know the levels

What about nitrate? How often do you do water changes and how much?

Imo medication should only be used as a last resort and if you know exactly what you are treating because it can be very hard on the fish. I also don't see any visible symptoms of disease. Did the guppies that passed away exhibit any symptoms or change in behavior?

And water changes I do once a week, a third of the tank

What are your exact parameters? His fins look clamped which is usually a result of stress.

Beside his fins, guppies are social fish. They enjoy the company of other guppies. Once you rule out water quality I suggest getting more guppies to keep him company.

Ok, so I went to an aquarist store and they gave me this solution that is supposed to get rid of any potential No2, No3 an NH3, as well as some heavy metals. I put my fish into a quarantine tank for the time being. I also got this testing kit and it shows that my nitrite level is 0, nitrate, however, was 25mg/l. I hope the solution they gave me will deal with it. I also bought two more male guppies and will slowly introduce them to my poor fish. Wish me luck that they'll get along and be healthy.
Thank you for all your advice and please let me know if there is anything else I should do!