I've never seen my fish do that before. That fella is about 10-11 months old, little bit smaller than his brother for some reason. They were both raised in the planted 55 gallon, so I don't think he's stunted, but something is up. First problem I've had with any of them. He's been normal until today. He's not inbred either.
He's clenching so he looks weird too. Could he be developing swim-bladder problems? I once had a molly with a swim-bladder infection purchased at a store, never had one of mine get it though, I try to keep things nice and clean without over-doing it and stressing the system. Wasn't quite like this though...it bobbed a bit, but this guy looks like he's having a constant seizure.
I change out about 30% once per week, half gravel vac so I don't over-do it. When it's hot the changes are bigger, often adding water or letting it get a little low, so it's usually something like 15-20 gallons. Tank is 55 gallons, about two years old. Has seven-eight adult fish, the rest are juveniles soon to be in new homes, though I'll be keeping three platies, swordtails and female mollies.
Tank parameters are:
Test kit used: API Master Freshwater Testing Kit
Haven't had any spikes or problems in more than a year and a half. Problem I had a year ago last January was and internal parasite brought in on a fish I neglected to QT(stupidest thing I've ever done I think), I have not done that again since and once it was gone, I've had no problems. These fish were not born yet. The fish is a Butterfly mix molly(likely with a gold dust or short-fin, but unknown).
I haven't added any new fish at all in the last five months, no new plants. I did however, put in a catappa leaf this morning for young shrimp and babies, I soaked it overnight and rinsed it well before adding it.
Lighting schedule has been about six or seven hours lately, it's been so hot(though they do get some very indirect light from the window on the other side of the room, nothing directly at it or even bright at all, for another two hours or so). Usually they get 8-9 hours with lights on.
I feed them Aqueon Tropical Flakes, blanched fresh veggies like cucumber and zucchini, romaine lettuce, the occasional bit of boiled kale. They also get frozen foods like Hikari's Spirulina brine shrimp, Emerald Entree, mysis shrimp, glassworms when I can get a hold of them. More veggie than anything.
I feed them small amounts twice a day so they aren't ravenous every meal.
Anywho, I have a video of it. One of my fish is just jiggling like a serious shiver going on. I have video for you since it shows much better than I can explain.
Hope I covered all the details requested for the disease page. https://www.youtube.com/watch?v=8cIPguJgQrs