Appears you have been struggling with platy's since Sept. of last year.This is not uncommon when we begin trying to adjust water to suit the fish, rather than simply purchasing fishes that will thrive with the tapwater we have whatever the pH or hardness is.
When I suggested last September, using crushed coral to increase the hardness and thus raise the pH , I should have also mentioned, that as the crushed coral get's covered with algae,mulm,bacteria,dirt,etc, it becomes less effective and should be cleaned once a month with hot tapwater or replaced.
Could be why your pH is now wanting to return to that closer to what comes out of the tap.
I suspect that if you were to leave five gallons of tapwater to set overnight,that the PH may be closer to 7.2 than the 6.8 right from the tap without setting overnight but you can try this and see.
Ammonia is indication of too much food being offered too frequently and some going uneatean or,, something decaying in the tank,or organic build up in substrate. I might try feeding tiny amount of food for no more fish than you have ,once a day. Would see that no one else is also offering food.I recall much younger years when My three sister's unknown to me,,were also feeding my fish ,and water quality issues were baffling to me until this was discovered.
Might also suggest not replacing filter cartridges until they begin to literally fall aprt from swishing them around in old aquarium water you take out during weekly water change.Would not clean this cartridge in tapwater for any reason.This will help save money on cartridges,and prevent you from throwing out beneficial bacteria that works to consume afore mentioned ammonia, and or organics that may be lying about on substrate.Might also consider vaccuming areas of the substrate wher there are no plant's more often and try and vaccum different areas each time .
Might also test tapwater for Ammonia to see if this is present and would strongly urge you to consider dechlorinator such as PRIME if you are not already using it.
These are some things for you to consider but were it me,, (and it ain't) I would consider keeping fishes that enjoyed soft acidic water you are blessed with. Things become way easier and hobby will be more enjoyable.