Fish that bloat/ balloon up suddenly and die a few days later have an internal bacterial infection. These are pretty hard to treat.
The guppy that was shimmying (swimming under the wood but not going anywhere) either has a protozoan infection, or there is a water quality issue (your results say otherwise), or the GH is too low for them.
Livebearers (guppies, mollies, platies, swordtails) naturally occur in water with a GH above 200pm (250ppm for mollies). If the water is soft they are more prone to health issues and bacterial and protozoan infections.
What is the GH (general hardness) and pH of your water supply. This information can usually be obtained from your water supply company's website or by telephoning them. If they can't help you, take a glass full of tap water to the local pet shop and get them to test it for you. Write the results down (in numbers) when they do the tests. And ask them what the results are in (eg: ppm, dGH, or something else).
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Before you add anything, do a 75% water change and gravel clean the substrate to try and remove nay gunk in it. 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. Wash filter media in a bucket of tank water.
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You can add rock salt (often sold as aquarium salt), sea salt or swimming pool salt to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres 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.
The pleco won't be happy with salt but will tolerate it for a couple of weeks.
If you only have livebearers (guppies, platies, swordtails, mollies), goldfish or rainbowfish in the tank you can double that dose rate, so you would add 2 heaped tablespoons per 20 litres and if there is no improvement after 48 hours, then increase it so there is a total of 4 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, 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 but the higher dose rate will affect some plants. The lower dose rate will not affect plants.
After you use salt and the fish have recovered, you do a 10% water change each day for a week. Then do a 20% water change each day for a week. Then you can do bigger water changes after that.
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Your pleco needs a bigger tank, preferably without livebearers. Plecos come form soft acid water, whereas guppies come from harder water with a pH above 7.0. For best results you would be better off keeping these species in separate tanks so you can give them water that is suited to their species.