The orange fish is a swordtail not a guppy, but none the less, it has a bad case of fin rot on the tail.
Triple sulpha used at the dose of 1 tablet per 20 litres (or whatever the maximum dose is) of water should help.
You can also add salt (sodium chloride) at a dose of 2 heaped tablespoons per 20 litres (5 gallons) of tank water. Keep salt in there for 2-4 weeks. the salt won't harm the fish, plants or filter bacteria.
You can use salt and triple sulpha together.
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Before you treat the fish do the following.
Wipe the inside of the glass down with a clean fish sponge.
Do a 75% water change and gravel clean the substrate. 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 them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.
Increase surface turbulence/ aeration when using salt or medications because they reduce the dissolved oxygen in the water.
