Hope this helps: :) https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm

Welcome!
Greedy fish can look more rounded and may account for a tummy getting bigger.
Often, a livebearing fish develops a definite gravid spot and I'm not seeing this from the photo. [She may well be female but, if you check pictures on line to determine a male's rod-shaped gonopodium versus a fan-shaped anal fin, you'll definitively know whether or not she is female. If she is, then there's an extremely high chance she's pregnant if she's been in the presence of a male fish.]
Erratic swimming may be accounted for by a number of issues - check ammonia, nitrite and nitrates with a liquid-based test as impaired water quality is the usual cause, or possibly by inappropriate water hardness (eg too soft when they require hard water).
Hope that helps.

...and also, if you could let us know the dimensions of the tank too (width/length of tank, depth front to back, height), that would be helpful, as it may help us refine our suggestions accordingly.

Yes, I'd say s/he's a comet.

Welcome!

As you're probably now aware, both ammonia and nitrite are toxic to fish, so the key will be to do whatever frequency and size of water change you need to in order to get and keep them at 0 - this may even be a couple of 40% water changes per day. Read and follow the advice in this https://www.fishkeeping.co.uk/articles ... ammonia-nitrite-spike.htm , and hopefully your remaining 4 cardinals will survive the experience.

In due course (maybe a month or two from now), consider adding more cardinals to the shoal - but keep monitoring ammonia, nitrite and nitrates for the next couple of months after that too.

Glad Rocky is still alive, still looking so handsome, and unperturbed by water changes.

Is there any possibility that he's struggling with the flow from the filter and is looking for a rest place? I see it's an external filter, which I've never had, and don't know / can't tell whether the flow is able to be reduced or not. What other decor do you have in there that he might be able to rest behind? Additions/alternatives to adjusting the filter flow include the links previously listed plus some tall silk plants.

He's lovely (and good photo).

As the spot hasn't changed, no more have appeared, and he's eating and behaving as usual, I don't think it's anything to concern yourself about at the moment. I would simply continue to monitor the situation, and ensure that your water quality is *consistently* at 0 ammonia, 0 nitrite and <20 nitrates (or no more than 20 above tap water nitrates level) to optimise the possibiity of it disappearing or at least not getting any worse.

If there is any change at all, let us know, and we'll advise further.

He's incredibly handsome! Such a strong, vibrant colour.

That behaviour does seem strange, although glad he was only like that for a couple of minutes. I know bettas have various querks, though, and often like to go somewhere for a rest, so perhaps that was his best option - other options, and so that they can more easily access the water surface to breathe, include https://zoomed.com/betta-bed-leaf-hammock/ or https://zoomed.com/floating-betta-log/

I had a look at Masterfisch the other day after you mentioned it - you're correct in that the website gives the implication that they're shipped from the UK but not specifically stating it. The name sounds / is spelt the German way, so I expect it has links with Germany - I have no idea whether this accounts for the convoluted journey he took. I'm a bit surprised at UPS being the couriers, though - take a look at https://www.ups.com/gb/en/help-center/ ... ort/prohibited-items.page which explicitly states live animals. As far as I'm aware, only APC couriers is DEFRA-registered for the transportation of live animals.

There's a few possiblities I can think of but, in order to get a better idea of the situation, it would be helpful to know:
* how long this has been present;
* is the fish behaving any differently or appetite different to usual;
* what your readings for ammonia/nitrite/nitrates have been lately;
* filter maintenance regime;
* tank temperature;
* PH and water hardness levels;
* size of tank and other occupants.
Thanks. :)

I'm not sure this is necessarily dropsy. However, something is clearly not right, what with the swelling (which in turn has caused distended scales over the lump) and one side of the body generally swollen.

Whatever it is, eSHa 2000 sounds wise.

I would omit the salt at this stage, though.
Aquarium salt = sodium chloride = mild antiseptic (and thus fine for the original lump/spot).
Epsom salts = magnesium sulphate = draws fluid out from the body.

If the eSHa 2000 doesn't work, even after an extended treatment dose (5 days rather than 3 days), then Epsom salt dips/baths may be worth a go to try to reduce any outstanding swelling.