Hi out there,
Nothing toooo concerning going on, but you hear horror stories about someone's entire stock dying "out of nowhere." I figure I'd check in with some more experienced folks in case these are actually signs I should be wary about. Thanks in advance to anyone who reads through all of this.

20 gallon high
HOB filter
Fairly well planted
5 burmese rosy loaches, 5 male endlers, 2 female platy (+ one very tiny fry that wasn't growing in a different tank, so I threw him in here the other day to see if he'll do better), 2 adult assassin snails and several baby assassin snails.

So, this was my first batch of rosy loaches, and I fully intend to bring them up to their preferred grouping of 8-10. From the start, they've been very bold little fish in my community tank, and I've had them for a few months. Since they didn't seem "bothered" (for lack of a better word) by their low numbers, I didn't rush back to the store after quarantining the first batch to get more. However, just for the past few days, I've suddenly started seeing much less of them. They're usually all pretty visible, or if not, they all certainly come out for feeding time. I can tell them apart, so I know none of them have died, but I'm only seeing one or two come out at a time for the past couple of days, then it's back into the plants. Even at feeding time yesterday only a couple made an appearance. They've really colored up since I've gotten them, so the hopeful part of me is thinking maybe they're back there reproducing (3 males, 2 females). Alternatively, I guess there could be other behavioral changes related to their maturing going on. Maybe now there's been some change in the group dynamic and I'd see less hiding behavior if I got more.
Last night, I noticed 4 baby assassin snails floating on the surface. I had seen one do that here or there, but it seemed odd to have 4 suddenly do it at once.
I tested ammonia (0 ppm) and nitrate (< 5ppm) then.

Other things of note:
* I had added a sponge filter to this tank before I added the burmese rosy loaches. It was in there with them for the past couple of months until Monday night. My amazon frogbit had all but disintegrated since adding this extra turbulence, so I decided to move the sponge filter to a different tank to give my frogbit a chance. Could the change in aeration or filtration be affecting them?
* It just got pretty cold this week. The thermometer in the tank reads a pretty consistent 78F, but for the first time ever I noticed heat waves coming off of the heater, so I'm thinking they may feel soooooome variation in temperature. Also, I'm sure the heater's current placement isn't optimal.

Photo from just now. Excuse the algae.
tank 12.1.18.jpg and poor qual.