Guppy-possible fin rot

Looking for advice please.
My daughter was recently given guppies as a gift.
We've never had fish before but we were recommended to set up the tank first, had the water tested (it was fine), get one guppy, test the water again then get the rest. We did all that. We use everything recommended and now have 5 guppies(all male). They had their first clean out yesterday (emptied approx 25% of tank, treated some fresh water then added it in slowly, making sure temp etc all stayed correct). We also used aqua salt which we hadn't originally used (used maintenance dose as on instructions. It was recommended by shop to use this).

We have one that's silvery with a spotty tail. It's nody seems to gradually be going darker. Yesterday we noticed it's tail was cut-it was like it had a slice through it. I've been keeping a close eye and its gradually becoming more ragged looking.
The fish still seems to be eating etc as normal.

Is there anything I can do to help the poor wee guy? Is there anything that can make him better/worse? Would this be likely to be something I have done?
I've become quite attached to them all already.

Thank you.

Pictures of the fish?

Fin rot is caused by poor water quality.
Treatment is normally doing a 75% water change and gravel cleaning the substrate every day for a week, and maybe adding salt. But post pictures before adding anymore salt.

Test the water for ammonia, nitrite, nitrate and pH. Post the results in numbers here.

What sort of filter is on the tank?
How often and how do you clean the filter?

Here is a photo (top fish).
I haven't cleaned the filter as we have had them just a week now. It's an aqua internal filter 100 for our 25L tank
I only have dipsticks which I will also post a photo of.

Some questions

Did you cycle the tank?
Does it have a filter, heater, and/or live plants?

JIP2023 said:

We also used aqua salt which we hadn't originally used (used maintenance dose as on instructions. It was recommended by shop to use this).

Click to expand...

How much salt did you use and what brand is it?

JIP2023 said:

we were recommended to set up the tank first, had the water tested (it was fine), get one guppy, test the water again then get the rest. We did all that. We use everything recommended and now have 5 guppies(all male).

Click to expand...

How many days did you leave it running before you put the one guppy in, and how much time did you leave the one guppy in before putting the others in? This is not a good way to cycle the tank and it will make the single guppy very stressed.

Yes it has filter, heater and live plants.
We had it all up and running for a few weeks before getting any guppies.
The store told us to get one, wait a week, test the water and then get more. So it was a week.
We used aqua care tonic salt 1g per litre (which is on the packaging for general aquarium tonic)

You need to get an ammonia tester...that test kit doesn't include one. Typically the strips aren't accurate. I'd get a liquid test kit, specifically the API Liquid Master test kit. It's a little pricey but well worth it, it's super accurate and has over 800 tests.

Do not trust anything the "store" tells you. 9 time out of 10 it is not true, if you have a question or concern ask on a forum, like you did.

Keep the salt in for a week or 2, or until the fin is fully healed. What's weird though is when one of my guppies got a split in its fins a few days later it fused back together. But hopefully salt and clean water will o the trick.

25 litres is pretty small so as others have said, you need to be doing much larger water changes. You wouldn't routinely add salt to an aquarium if your fish are healthy, once your fish has healed I wouldn't add any more salt as it's not necessary.

Thank you. This is really helpful.
How often should I clean the tank at the minute, and should I do everything each time (filter, gravel etc)
Thank you again

The most you need to do for a gravel is an occasional gravel vac, never rinse it or scrub it, and for the filter you can clean the filter media in TANK water once a month or when the flow slows.

@Colin_T is good at explaining salt and water changes for fin rot, so hopefully he will help. When was the last time you did a WC?

The missing piece of tail looks like a bite. One of the other guppies probably bit it or the fish got caught on something like a plastic plant and ripped a piece off.

Clean water and salt should stop any infections.

The darker body colour can be the fish's natural colouration, which can vary a bit depending on time of day (day or night) and the fish's mood.
If a fish gets a cream, white or grey patch or film over its body, that is a concern.
If a fish gets red areas on the body that is a concern.
If the muscle tissue under the skin goes cream/ white, that is a concern. But the fish's colour looks ok now.

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Because it's a newly set up aquarium, you should reduce the feeding to about once every couple of days and do a 75% water change and gravel clean the substrate 4-8 hours after feeding. The fish won't starve but the less food going into the tank, the lower the ammonia and nitrite levels will be. In about a month's time when the filter has established, you can increase feeding to once a day.
Make sure any new water is free of chlorine/ chloramine before it's added to the tank.

The new water doesn't have to be exactly the same temperature. If it's within a couple of degrees it's fine.

You should gravel clean the substrate any time you do a water change. If you have live plants in the tank, leave a couple of inches of undisturbed gravel around the base of them but gravel clean the rest of the tank.

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Do not clean the filter for the first 8 weeks unless it gets really dirty and isn't pumping much water. After that time you should clean the filter at least once a month.

To clean the filter, turn the pump off at the power point and unplug it. Take the filter media/ materials and squeeze them out in a bucket of aquarium water. When the media is clean, put it in the tank while you clean the filter case and motor. The filter case and motor can be rinsed under tap water. Reassemble the filter. Put the old filter media back in the filter. Plug it in and turn it back on. The bucket of dirty water can be poured on the lawn outside.

The motor on the filter should also be taken apart to clean the impeller in it. The impeller is a magnet with a couple of plastic blades on one end. There should be a steel or ceramic shaft that goes through the impeller and there are usually rubber grommets on either end of the shaft. Be careful not to lose any of these pieces because the pump won't work properly without them. The filter should have and instruction book on disassembling the motor so it can be cleaned. If it doesn't post some pictures and we will try to work it out.

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SALT
You can add rock salt (often sold as aquarium salt), swimming pool salt, or any non iodised salt (sodium chloride) to the aquarium at the dose rate of 1 heaped tablespoon per 20 litres (5 gallons) 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.

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, Bettas & 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, fish, plants, shrimp or snails.

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.

When you first add salt, add the salt to a small bucket of tank water and dissolve the salt. Then slowly pour the salt water into the tank near the filter outlet. Add the salt over a couple of minutes.

Thank you so much. This has been so helpful