Tank is starting to go bad

[Kimm87]

day 1 of this tank every thing was fine
Day 2 1 guppy was swimming alone and hiding
Day 3 got hornwort for tank shy guppy started acting normal again
Day 4 noticed a differnt guppy had sunk in belly not eatting or interacting
Day 5 that fish was dead this morning another fish has a chunk missing from its tail possibly fin rot?

Im not sure whats going on with this tank i noticed on day 2 the dead fish was swimming with its mouth open and had an odd body shape, I didnt remove it figured what ever was wrong with it the others had been in the same tank all this time at the store so it wouldnt matter.
The fish with the chunk missing has a whole section missing from the middle section of so it looks like he has a split fin it has some white around the area .It wasnt like that last night

 

[Lunar Jetman]

Can you let us know how long this tank as been set up? What size is it? What filter have you got? What are your water parameters?

 

[Colin_T]

pictures, pictures, pictures

if in doubt, water change it out
if you lose any fish, or they look sick or are acting unusual, do a 75% water change and complete gravel clean each day until they look better or you can figure out what is going on.
Make sure any new water is free of chlorine/ chloramine.

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What else is in the tank?
If there are only guppies, then add some rock salt, sea salt or swimming pool salt, or aquarium salt if you want to buy that.
For guppy only tanks use 2 heaped tablespoons of salt per 20 litres of water. If there is no improvement after 2 days, add another 2 heaped tablespoons per 20 litres.

If you have other types of fish that are not livebearers, then use 1 heaped tablespoon per 20 litres and add another heaped tablespoon if no improvement after 48 hours.

Keep the salt levels high for at least 2 weeks but no more than 4 weeks.

To work out the volume of water in the tank:
measure length x width x height in cm.
divide by 1000.
= volume in litres.

when you measure the height, measure form the top of the substrate to the top of the water level.

there is a calculator/ converter in the "How To Tips" at the top of this page.

 

[Kimm87]

We got the fish saturday its a 10g the filter is a aqueon quiteflow or somthing but its stuffed with sponge and biomax ceramic medi. Idk what the parameters are we had the water tested at petsmart Monday and everything was fine. I dont have a test kit and the soonest i will be able to get 1 is the 10th if i have any money left from my check.
I dont think its a water quality issue there is a ton of plants in there and established media from my frog tank

 

[Kimm87]

This is all going to have to wait till this afternoon i have to leave in a few minutes
Yes there are just guppies and plants in the tank. I added a dose of prime to the tank just incase hooefulky they are fine when we get home in like 3 hours



We are headed home now i got non iodized sea salt.
Do i do a water change and then add the salt and do a water change again in 2 days with more salt?

I added salt do i need to raise the heat? I have a 500w adjustable temp heater i can add right now theres a 200w preset heater in there.
Have you ever tried to take a picture of a 1/2 inch long guppys tail with a low end cell phone? Its impossible how ever in the few hours we were gone the tail has further deteriorated and now has a hole in it.