Hi there - sounds like my 2 adult platys have a similar problem i.e clear poo since yesterday, listless and either resting at the bottom of the tank or gulping air at the surface, not eating.
Also in tank are 1x5 week old fry + 5x1week old.
Recent events: put in large live plant a week ago for new fry to hide in (bought at PetsatHome and had been in tank with guppy fry), added heater a couple of days ago since freezing weather (tank light previously seemed to suffice in keeping tank 22-26 degrees -currently at 26), also started using liqufry no.2 a couple of days ago.
Tank - Aqua One - 27 L altho probably more like 24L with plants and gravel etc
Filter - Fluval U1
Tank stats: ammonia 0, NO3 0-10, NO2 0, pH 6.8
Tried squashed peas this morning in case they are constipated (until yesterday they ate anything I put in for the fry as well as there own food - soaked pellets - but they aren't interested. Have put in Mixozin 10 drops in this morning in case it's an infection.
Have read the rest of this thread about worms etc - no visible evidence of infestation other than clear poo. Tried to find supplier for levamisole and avitrol plus but only seems to be available from USA. I live in South Yorkshire. I don't have a quarantine tank
What do you advise now?