Hi and welcome to the forum
If you have a filter that containers filter pads/ cartridges that need replacing, you can add sponges to it from other brands of filter, and the sponges will hold the beneficial filter bacteria and won't need to be replaced. You can buy sponges for numerous brands of filter (I used AquaClear but there are others too), and cut them with a pair of scissors so they fit inside your filter.
You can also get round/ cylindrical sponges for some brands of internal power filter and these fit over the intake strainer of most external power filters. The sponges get squeezed out in a bucket of tank water and re-used. Sponges will last for many years and only need replacing when they start to fall apart.
With filter pads, you can squeeze them out in a bucket of tank water and re-use them. If they have granules in them, make a small cut in the top of the cartridge and tip the granules out and throw the granules away. Then squeeze out the cartridge and re-use it.
Filters should be cleaned at least once a month, and every 2 weeks is better. However, if the filter is less than 6 weeks old, do not clean it because you can wash out the beneficial filter bacteria that keeps the water clean.
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How often do you do water changes and gravel clean the substrate?
I recommend doing a 75% water change and gravel cleaning the substrate once a week. If you live in an arid climate or have water restrictions, do it every 2 weeks.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.
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Any chance of a picture of the fish?
If the pictures are too big for the website, set the camera's resolution to its lowest setting and take some more. The lower resolution will make the images smaller and they should fit on this website. Check the pictures on your pc and find a couple that are clear and show the problem, and post them here. Make sure you turn the camera's resolution back up after you have taken the pics otherwise all your pictures will be small.
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Guppies are notorious for having infections when you buy them and should be cleaned up a bit when you get them. They either have protozoan infections or bacterial infections. Both respond well to salt. They also carry intestinal worms and gill flukes, the flukes can be treated with salt and the worms needs to be treated with other things.
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.
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 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.
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The following link has information about what to do if your fish get sick. It's long and boring but worth knowing. I recommend printing it out and reading it in bed to help fall asleep.
https://www.fishforums.net/threads/what-to-do-if-your-fish-gets-sick.450268/
here have any info on where to get some quality fish online that would be great... my LFS has fancy and endler guppies for 4$ (US) per fish... I would be willing to pay at least that much. Next I need to figure out how many guppies my tank can hold. (15gallon) 
