Plecostomus need big tanks (6 foot plus) and lots of big water changes and gravel cleaning the substrate to remove the waste they produce.
Guppies are inbred and weak these days. They are also regularly infested with intestinal worms and gill flukes and should be treated for these as soon as you get them. They also need water with a GH (general hardness) around 200ppm and a pH above 7.0. Local bred guppies are generally healthier than fish from Asia or fish farms.
Betta splendens are inbred too and are not good quality. If you can find good quality fish, they are easy to keep and should live for 3-5 years, but most of the stuff sold in shops are inbred and genetically weak.
GloFish tetras (depending on the actual species) can be easy to keep. Get a group of 10 or more and have lots of plants and they are usually fine. Some tetras will bite the fins on slow moving fish or fish with long fins and they should not be kept with guppies or Bettas.
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All freshwater aquariums should get a partial water change and gravel clean once a week. You can get a gravel cleaner from any pet shop or make your own from a plastic drink bottle and length of garden hose. There is a picture of a basic model gravel cleaner on the following link. It's about half way down the page. You can look on YouTube to see people using gravel cleaners.
New water should be free of chlorine/ chloramine before it's added to the tank.
https://www.about-goldfish.com/aquarium-cleaning.html
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Filters should be cleaned at least once a month. However, if the filter is less than 6 weeks old, do not clean it, wait until it is 6 weeks old before cleaning it for the first time. Then do it monthly after that. Wash the filter materials/ media in a bucket of tank water and re-use the media. Tip the bucket of dirty water on the garden/ lawn.
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Wipe the inside of the glass down with a clean soap free sponge, once a week before you do the water change.