Need urgent advice. My bettas dorsal fin has disappeared in the 72 hours I’ve had him.

I don’t know exactly what is wrong or how to treat it. Need advice urgently. Any help is much appreciated.

Housing:
How many gallons is your tank? 5.3 (24 litres)
Does it have a filter? No
Does it have a heater? Yes
What temperature is your tank? 79
Does your tank have an air stone or other type of aeration? No
Does your Betta have tank mates? No
Food:
(He's not ate since I got him, but King British/ Love Fish pellets)
Maintenance:
(I've only had him 3 days so I've not changed the water yet)
What brand of conditioner? Love Fish Tap Safe
Water Parameters:
Ammonia: 0
Nitrite: unknown
Nitrate: unknown
pH: 6.5
Hardness (GH): unknown
Alkalinity (KH): unknown
Symptoms and Treatment:
When did you first notice the symptoms? 24 hours ago. (Yesterday I noticed his dorsal fin looked like it had been torn in half, but my parents thought he always looked like that and told me to just keep an eye on it)
How has your Betta’s appearance changed? His dorsal fin has nearly disappeared, the colours have started to fade at the top of his tail, the base of the dorsal fin is turning white, and he has a small white/transparent lump on his chin. (This all happened in less than 12 hours.)

How has your Betta’s behavior changed? Very inactive yesterday, but quite active today.
Is your Betta still eating? He hasn't eaten since I bought him 3 days ago.
Have you started treating your Betta? No
Does your Betta have any history of being ill? Unsure (only bought him 3 days ago).
How long have you owned your Betta? 3 days.
Was he or she ill or suffering some sort of damage when purchased? Possibly. The back of his dorsal fin looked a bit different to the rest of his fins but the guy at the pet shop said he was a crowntail so I assumed that’s just how his fin was. (I originally thought he was a halfmoon but I’m not very familiar with bettas so thought he might be a cross between halfmoon and crowntail). Now I’m guessing that it was actually fin rot on his dorsal fin.

Probably fin rot, it is quite common is bettas. Either call the LFS and ask if you can exchange for another or try to cure it with meds and regular water changes depending on what the meds say

if you can get your self a full test kit and proper betta food that will help a lot but otherwise just keep doing water changes about 50 % every two days should be sufficient. i also dont recommend taking it back as it will most likely die very quickly.

also is the tank planted and what decor do you have in the tank.

I agree with Sean, test results on the water will be very useful. (Ammonia's most useful right now, but Nitrate and Nitrite readings would be useful as well)
You said you didn't have a filter, did you cycle the tank before you added him?
For right now, as Sean said, keep doing water changes. It does look like fin rot, which is usually caused by poor water conditions. Luckily, it's very treatable c:

I also see the start of body rot. Unless treated properly and immediately it will be fatal.

but the guy at the pet shop said he was a crowntail s

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The guy at the shop wouldn't know his elbow from his backside.


Please read this

Aquarium Salt is sodium chloride, which is found in most fish sections of stores and can also be found as non-iodized (!!!) table salt or ice cream salt. Put simply, it's mostly just salt! It is very easy to acquire and inexpensive. It is wise to have "AQ" salt on hand when caring for fish due to its nearly limitless uses.

What's AQ salt do?:
AQ salt does quite a lot! In freshwater, it can make water brackish (salty) for fish like mollies, can help fight off existing infections, prevent infections from coming back, can help parasites like Ich and Velvet not "stick" to fish, help ease fin rot, help heal wounds, and help ease columnairs along with other medicines.

What does this mean for my fish?:
This means a lot!! AQ is super versatile and easily the least expensive, most widely effective medicine you can buy. It means you can heal things from fin rot to parasites to wounds. That's a big deal, especially when these ailments tend to come on fast and need prompt treatment to be kept from being deadly.

How much of this salt per gallon do I use?:
For AQ salt, you can use 1 TABLEspoon per gallon of water (3 TEAspoons).

How long does the betta stay in this mixture?:
This mixture is reported as considerably more dangerous and more uncomfortable for the fish, so it will have less time than Epsom dips. Dip the fish for 5-8 minutes, depending on severity (for example, fin rot less, columnaris and ich more). Do not exceed this time!

How long should my fish be left to acclimate before going back into its tank in this mixture?:
Acclimate your fish (see below) for 2-3 minutes. This time frame is less than Epsom because we do not want the fish to be further exposed to higher doses of salt for too much longer after the bath.

How to prepare (any) dip for your fish:

Prepping for the dip is the same for either salt type. You will need:

• a clean fish-only 1 gallon container
  
• a smaller container
• Water conditioner
• a measuring spoon
• thermometer
• net
• your undivided attention!
• watch/timer/alarm
• salt per your needs

Here's what you need to do:

1. Fill your gallon container very fully with clean, treated water. Make sure the water is the same temperature as the water the fish came from in its tank via the thermometer. This prevents temperature shock.
2. Add the salt per recommendation and stir it until it is fully dissolved.
3. Get a second container with 1/4 salted water like your 1 gallon tank, and the rest (3/4) with tank water (this is the "reviving station").
4. Carefully get your fish from its home and gently put it into the water. You must make sure that the fish does not pass out! You can tell if a fish has conked out if it is no longer breathing (look at its gills, by the eyes) or if it lists to the side and becomes still. It may knock out due to the sudden change in salinity (saltiness) of the water, but can be revived.
5. *If the fish passes out or becomes extremely stressed, remove it! Carefully but hastily put the fish into your reviving station to revive. Then, put it into its home again and try the dip another time.
6. If the fish is not super stressed and does not pass out, be very attentive during the recommended time in the dip as the fish may jump due to discomfort or stress. Pay close attention to how long the fish has been in the dip and do not exceed the time!
7. Once the fish is finished, put it into the reviving station to adjust back to more normal water parameters. Then, without pouring the salty water into your tank, put the fish back into its home to recover.

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Medication options.

• Maracyn II (Two) is an antibiotic that attacks gram-negative bacteria, which is typically the bacteria that causes this kind of rot. It can be found in most places that sell fish supplies, like Petsmart. It is made by Mardel.
  Update: Unfortunately, the Maracyn series is no longer being made. Make sure yours is within date if you have it or are able to still buy it.

• Jungle Fungus Eliminator, Jungle Fungus Clear is a good product for treating aggressive rot, as well as many other diseases. The two downsides to this medicine are that it changes the water's color DRASTICALLY (blue/green) and, when purchased in tablet form, it is made for 10gallons/tab. To shorten the dose, use scissors and scratch along the tablet in half, quarters, etc, then break. Like Maracyn, it is widely available, even in Wal*marts, as well as Petsmarts, etc. Made by Jungle, and can also be found in powder form.
• Erythromycin (try: API Brand) is an antibiotic that can work against fin rot. like Jungle Fungus clear above, it may be hard to dose in tablet or powder form if you are dosing for a smaller tank. It should not discolor your tank's water or decor. This item is expensive: USD ~$17 and can be found at Petsmarts, and similar stores.
• Tetracycline (try: API Brand) is an antibiotic that can work against fin rot, but may not work as well as other medicines. If you can get it, try it. Please note potential issues with dosing as stated above. This item may discolor your water, but should be okay for decor. This item is expensive: USD ~$16 and can be found at Petsmarts, and similar stores.
• API Fungus Clear is a slightly less expensive option for treating rot. Please see potential problems with dosing as noted above. This product may slightly discolor your water but should be okay for decor. This item is of moderate price: USD ~$12 and can be found at Petsmarts, and similar stores.
• Other medicines, such as minocycline, Kenacyn/kanamycin, tetracycline, and many others are available under various names, sometimes generic ones. They can be found in fish stores, usually. You can also try your hand at Amazon.com if you feel comfortable with waiting for shipping.

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As Nick said you never listen to anyone at a petshop. Ever.

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This is why I shy away from long finned bettas, they seem to be far more prone to finrot. The others have already covered what you should do, water changes and probably a little something more such as an anti-fungal med.

You should also look into getting a small filter and perhaps do some reading up on the Nitrogen cycle. This always seems to play a part in fish becoming ill. I blame the people who were in charge of the bettas at the petstore, the ammonia and nitrites were probably through the roof in the betta cups.

When finrot is bad the fins will deteriorate within a couple days. I lost my first betta in a similar way. At this point I don't feel that simple water changes will do it. You probably will need to grab some meds.


Edit: He is not any part crown tail, but looks like a halfmoon/delta to me.