I've got 5 platy and 8 zebra danios in a 125l tank and one of the platy is looking increasingly 'deformed' in shape.
Basically a normal platy is pretty much straight from the top of its mouth up to its dorsal fin, but this one is more curved in shape from its mouth to the dorsal fin, and it definitely appears thinner than the others.
Now, I've done some digging and I've seen all kinds of reactions from 'it's fish TB so panic', to 'it's just genetics and inbreeding'. I've heard of Mycobacterium before, but looking at the symptoms of other fish, they have disjointed spines and ulcer-type lesions so I'm not convinced it's that.
How old is the platy? It's possible it could simply be a symptom of old age. [I get the impression that 3-ish is considered the current life expectancy of a platy.]
Is it eating enough? The curved shape you describe actually seems less evident to me than the flatness of the belly.
How is your water quality - ammonia, nitrite, nitrates in particular? Ensuring that they are at 0, 0, <20 (or <20 above tap water levels) at all times is crucial. As for water parameters, platies are generally hard water fish, so, if your water were soft, that might account for the problem.
I've had a lot of fish which have developed spinal problems in the past couple of years - curvatures when viewed from above or laterally, to the extent that the tail almost seems to 'break' ultimately before the fish's final demise, and also becoming increasingly emaciated. Coincidental or otherwise, I also had a young shoal of fish, many of whom developed spinal curvatures similar to the arch shape you describe before dying very prematurely. Whether or not the older fishes' problems were old age, TB (mycobacteria marinum which they'd be more susceptible to due to being old - and possibly brought in by the newer shoal of fish which may have been carrying it) or something else altogether is unknown - only a post-mortem would be able to definitively prove/disprove TB. I sought advice from a well-renowned expert recently and asked this specific question about TB - they pointed out that not all fish display symptoms or necessarily all the symptoms, so equally I wouldn't rule it out.
Whatever the cause is is probably neither here nor there - I would continue giving the same care/attention as usual. However, just in case it is TB, then I'd take extra care when doing tank maintenance to ensure no open cuts on your hands through which an infection may transmit - either postpone the tank maintenance until it's healed or wear gloves (although I've never succeeded yet in not ending up with water entering the gloves!). If you have a spare tank (and heater if you use one and filter), then that would be an added layer or protection for the fish - you could isolate this fish and any others displaying symptoms and keep them in it separately to avoid any further cross-infection; while most of the fish I mention have died in recent months, I continue to house one on his own in the spare tank and he's faring well (while the others are protected in the main tank).
Actually this platy is less than a year old and none of the others seem to be 'suffering' like her. She eats well and still comes to the front of the tank when I go near it. My quarantine tank has some harlequin rasboras in at the moment so i cannot isolate her, but I will keep an eye on her.
Water paramaters are the same story; 0 ammonia, 0 nitrite and too much nitrate. Nitrates out the tap, according to the water company, is between 18 (min) & 27 (max) and is the biggest problem I have with keeping fish. I've started replacing 30l of water, twice a week to try and get some kind of control over it because algae is becoming a real pain, not to mention the stress on the fish. Do you think it could be related?
Your aim will be to keep nitrates <42ish - hopefully the twice-weekly water changes will start to pay off. Certainly nitrates can be a cause of algae - https://www.thesprucepets.com/nitrates-in-the-aquarium-1381883 might be helpful - and can in turn pose health problems for fish. The sentence about the level above which nitrates can be harmful to fish is very outdated, though - nowadays, it's thought that levels of nitrates have 40 have an impact and increasing species are affected by nitrates above 20 .
Presumably all food is within-date, not been opened for a lengthy period of time, and there's a variety of food being offered? (Just thinking of my own gang of fish who have very particular food preferences each, ignoring some food but enjoying others.)
I think the best plan for now is to keep monitoring her, as you say.