Any trace of ammonia can become toxic in water with a high pH, and a pH of 8.2 is pretty high. Seawater has a pH of 8.5 and low levels of ammonia can wipe out entire marine tanks very quickly.
Nitrite is not a problem in water with a pH above 7.0. Nitrite can be an issue in acid water (pH below 7.0) but is much less of an issue compared to ammonia. Your pH isn't acid so nitrite is nothing to worry about. Plus it's 0ppm so is nothing to worry about.
Nitrate is very low and that is fine. As long as nitrate remains under 20ppm, you have nothing to worry about there. If it does go up, the quickest way to reduce ammonia, nitrite or nitrate is with a big (75%) water change.
Have you checked the well water for ammonia, nitrite, nitrate and pH?
You might also want to get the well water tested for anything and everything by a water testing company just to make sure there is nothing bad in it.
When you clean the filter each week, do you wash it under tap water or in a bucket of aquarium water?
You want to squeeze/ rinse filter media in a bucket of aquarium water and re-use the media. tip the bucket of dirty water on the lawn/ garden.
Does the filter have any white granulated substances in it?
If yes, this might be Zeolite, which adsorbs ammonia from the water and can interfere with the filter bacteria developing.
You can leave a 1-2 inch gap around the base of aquarium plants when you gravel clean the substrate. Most of their roots are small and usually grow within 1-2 inches of the base of the plant. Echinodorus (sword plants) have bigger root systems that can spread out 12 inches or more and it's a good idea to leave 3-4 inches of undisturbed gravel around the base of these plants when they are bigger.
Hard water (water containing lots of minerals) is fine for guppies, platies, swordtails, mollies, snails and most shrimp, so don't worry about the hardness (GH
If you wanted to keep tetras, angelfish or other softwater fishes, you would need to get a bigger tank and reduce the hardness by distilling or using a reverse osmosis unit. But for the guppies, you don't need to worry about the hardness.
The dead fish appear to have flared gills, and the black one has red around the body just behind the head. The flared gills can be a gill infection, gill flukes, poor water quality (unlikely considering the water tests ok), or poisoning from another source that isn't ammonia, nitrite or nitrate.
The red patch on the black fish is a bacterial infection.
I would add some salt to the tank and monitor the remaining fish. It can treat minor bacterial and fungal infections, as well as gill flukes and a few other issues. It might help and is reasonably safe.
If anymore fish get sick or act unusual, try to get a 1 minute video of them and upload it to YouTube, then copy & paste the link here. Likewise if they show any unusual colouration (red or white patches), take pictures and post them here asap.
If the shop offers you replacement fish, get a credit for them and use it later when things have settled down.
You can add rock salt (often sold as aquarium salt) or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres (5 gallons) of water. If there is no improvement after 48 hours you can double that dose rate so there is 2 heaped tablespoons of salt per 20 litres.
Keep the salt level like this for at least 2 weeks but no longer than 4 weeks otherwise kidney damage can occur. Kidney damage is more likely to occur in fish from soft water (tetras, Corydoras, angelfish, Bettas & gouramis, loaches) that are exposed to high levels of salt for an extended period of time, and is not an issue with livebearers, rainbowfish or other salt tolerant species.
The salt will not affect the beneficial filter bacteria, fish, plants, shrimp or snails.
After you use salt and the fish have recovered, you do a 10% water change each day for a week using only fresh water that has been dechlorinated. Then do a 20% water change each day for a week. Then you can do bigger water changes after that. This dilutes the salt out of the tank slowly so it doesn't harm the fish.
If you do water changes while using salt, you need to treat the new water with salt before adding it to the tank. This will keep the salt level stable in the tank and minimise stress on the fish.
When you first add salt, add the salt to a small bucket of tank water and dissolve the salt. Then slowly pour the salt water into the tank near the filter outlet. Add the salt over a couple of minutes.