What's wrong with my guppy

[kkllpp]

I have this guppy living with 3 other guppies in a planted 5 gallon.
Recently i observed him having stringy poop.
He also always had this rather curved spine.
Yesterday he was rather lethargic but still able to swim to the top and eat some pellets.
Just today I saw this lesion on his body and he is even more lethargic than before and seems to have lost appetite. Not sure if it is an injury or a kind of growth?
The other 3 guppies in the tank are very much active and healthy so not sure what's up.

water parameters:
nitrite: 0
nitrate: 10
pH: 6.8 (this is using a strip and i am not sure if it is accurate)
i don't have a means of testing ammonia right now bcos my api ammonia expired and is not accurate

 

[kkllpp]

update: the guppy passed

do still let me know what this may be so that i can keep a look out next time!

 

[Circus]

He could have been very weak in general. The ph seems very low for guppies, who like a high ph and lots of minerals in the water. With acidic and low ph water, many livebearers are prone to disease and the shimmies.
5 gallons is also very small for multiple guppy fish, so the water would get dirty quickly, and there is no where to get away when other males chase - which is even more stressful for the fish.

As far as the specific disease, I am unsure of a diagnosis.

 

[Colin_T]

Stringy white poop in fish is caused by internal bacterial infections, internal protozoan infections, or intestinal worms. The worms don't normally kill fish quickly but internal bacterial infections will kill a fish within 24-48 hours of showing symptoms. Internal protozoan infections usually kill fish within 1-2 weeks of showing symptoms.

The following link has information about stringy white poop in fish.
(263) What to do if your fish has Stringy White Poop. | Tropical Fish Forums

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If you ever lose a fish (it dies), or fish get sick, do the following.
Wipe the inside of the glass down with a clean fish sponge.

Do a 75% water change and gravel clean the substrate every day for a week. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the remaining fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.

Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.