Need a picture of the fish in focus.
What symptoms do they have before they die?
Is there a filter in the aquarium?
If yes, what type and how often do you clean it, and how do you clean it?
What is the ammonia, nitrite, nitrate and pH of the aquarium water?
You can buy test kits for this or take a glass full of tank water to a pet shop and ask them to test it for you. Write the results down in numbers at the time. If they say the water is fine, ask them for the results in numbers.
If the tank is newly set up (less than 2 months old), they are probably dying from ammonia or nitrite poisoning. This occurs when fish food and waste break down in water and produce ammonia. In an established aquarium there is good bacteria living in the filter and the bacteria eat the ammonia and convert it into nitrite. Another group of bacteria eat the nitrite and convert it into nitrate. You get rid of nitrates by doing water changes. In a newly set up aquarium there is no beneficial bacteria in the filter and the ammonia and nitrite levels go up and poison the fish. This will be made worse by the size of the aquarium (15 litres). The ammonia levels build up much faster in a small volume of water.
The colouration of the guppy is not a problem and fish can change a bit over time or when they are stressed, or even between day and night.
The fish itself (even though it's blurry), doesn't look like it has a disease. However, a disease might be more noticeable in a better picture.
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BASIC FIRST AID FOR FISH
Test the water for ammonia, nitrite, nitrate and pH.
Wipe the inside of the glass down with a clean fish sponge. This removes the biofilm on the glass and the biofilm will contain lots of harmful bacteria, fungus, protozoans and various other microscopic life forms.
Do a 75% water change and gravel clean the substrate every day for a week or until the problem is identified. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the 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 the media. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens so any medication (if needed) will work more effectively on the fish.
Increase surface turbulence/ aeration to maximise the dissolved oxygen in the water.
