What filter do you use? Aqua Internal Filter-100 (It came with the tank - not sure to replace with a Fluval?)
How long has the tank been set up? 15 days (10 days without fish; 5 days with fish)
What water tests have you done? pH - 7.8 (high?) Ammonia - 0-2.5ppm NitrIte - 0-0.25ppm NitrAte - 10-20ppm (high?)
When was the last water change and how much did you change? Not done yet, hasn't been a week since putting fish in tank
A list of all the inhabitants? 2 Platy's (wagtail) 1 Siamese fighter (gets on fine with other fish)
When did you last add fish, plants, ornaments or other tank equipment? Added one plant (valus gigantus) at time of putting fish in
The larger platy has developed a swollen belly - see pic. This is most visible after eating. I feed once a day. This fish almost immediately defecates after eating. By evening the swelling considerably decreases and it looks almost normal.
Is the swollen stomach a case of dropsy or stress? Though the fish appears to be acting normally, eating with vigour. It's not hiding away or hanging out at the bottom of the tank. (I notice that the healthy smaller platy constants follows - harrasses? - the larger one with the swollen stomach).
Thanks - very helpful for us that you've answered all the key questions.
Often, female platies in a tank in the presence of even one male for even as much as a minute can result in her being impregnated and able to store sufficient sperm to continue producing young for as much as 6 months. However, the fact that her stomach subsides in the evening eliminates that theory as an explanation.
It does seem as though the swollen tummy is dietary related, and I have also had fish whose tummies swell up after gorging themselves and later subside, and others who have become much larger than their shoalmates.
If I were you, I would: * feed the same amount of food but divided into two separate meals i.e. feed half the amount in the morning, and the remainder in the evening (to minimise bloating); * feed something like chopped pea (shell off, then the two halves chopped into small morsels) or daphnia (live or frozen - not freeze-dried which has a tendency to cause bloating) once or twice a week; * feed sinking or semi-sinking pellets rather than flakes, or pre-soak flakes so that they sink, to encourage the fish NOT to simultaneously gulp in air from the water surface while eating.
Good news that there are no other signs of anything else untoward but I would keep an eye on the situation. If the scales start to protrude like a pinecone, that's a definite sign of dropsy. If the fish starts to look permanently bloated and has trouble staying upright, then an Epsom salt bath can help.
Do weekly water changes of anything from 10-50% but keep monitoring water quality - the filter is unlikely to have been fully cycled after only 10 days but current results seem fine (which testkit brand do you have as that helps interpret the readings better). If there is any rise in ammonia/nitrite/nitrates before the end of each week, either do an additional or a larger water change.
Your filter ought to be fine. However, it looks as though it has cartridges containing carbon. Carbon stops medication from working properly - its only real use is for diluting/removing medication from the water post-treatment - so you'd need to get rid of them and replace with filter foam if dosing medication. Obviously this means that you would lose all the beneficial bacteria built up through cycling to process the fish's waste. The same would occur if you were to replace the cartridge if it started clogging up. What would be a lot easier for you would be to buy some filter foam and use this as the filter media but retain as much of the non-carbon filter media as possible, to avoid losing any beneficial bacteria being built up. I did this with a similar filter by cutting around the filter cartridge to retain the filter wool on the outside, then wrapping this around some filter sponge to impregnate it with the good bacteria, adding some Tetra Safestart (it and Dr Tim's One and Only are two products with the best reputation for effectiveness, albeit that can't be guaranteed, as there are so many factors involved in how effective these products actually are), and closely monitoring water quality for the next 6 weeks or so to ensure that all was going ok and doing extra or more frequent water changes if required to ensure that ammonia, nitrite and nitrates stayed at optimum levels.
Also, a 25-litre tank is too small for these fish. It's only big enough for a single Betta on his own. I would return/rehome the platies as they also have different requirements to the betta (he needs a higher temperature and softer water than they do).