FlytrapCare Carnivorous Plant Forums

Generally, terrariums are a bad idea for carnivorous plants, their needs are usually much more than a restrictive environment can provide. Since it's already done, lets move forward. My thoughts would lean to a combination of little things adding up.
First, that looks like a Mexican Pinguicula, and they do not like it nearly as wet as a capensis. It's likely being overwatered.
Next, in an aquarium, there's no drainage, so things are maybe getting waterlogged and possibly anaerobic. That's no good for either. If you're careful about the water as you say, there is still a stagnation potential due to no drainage other than evaporation.
Expanding on that, they evolved in two very different environments, and can be difficult to grow together. It would probably be better to separate them into conditions better suited for each.
Next, what's in that carnivorous plant soil? It's possible that is has nutrients and minerals in it, and these plants don't necessarily like that. Also, the preferred soil make up is very different for a Mexi-Ping than it is for a capensis.
Last, you didn't mention lighting, so it's very possible they're not getting enough light.
So growing conditions, water amount, water quality, media, and light could be insufficient in part or in total, creating an environment that is outside the goldilocks zone for these plants, and they're slowly dying. The Ping is just going faster.

I'd get the Ping out definitely, but, by the looks of leaves already falling off, it might be iffy. It'll probably fall apart when you dig it up. If so, stick the leaves into the media and try for strikes.
In either case, if a Mexi-Ping, a media with more perlite and sand for drainage would be better for the Ping, but less water. You still haven't said what it is, so I'm going off of my experience with Mexi-Pings. I have all my Mexi-Pings in trays, I gently top water until there's about 1/4 inch of water in the tray, let the tray dry up, and only water again when the top of the media is dry. Yours sitting in that wet media is outside its norm, and it's not liking it.
The Cape actually doesn't look too bad, but, again, terrariums are far from the recommended environment for most carnivorous plants. I'd pot it up and put it outside, on a sunny windowsill, or back under those lights, but not enclosed like it is.
Just my thoughts.

Chef gave you good advice! Terrariums just seem "perfect" for growing carnivorous plants, but...they ain't. The capes like it good an moist, a bit of water standing in their tray/saucer that is allowed to dry down and then be refilled again. Though capes are covered in "dew" they do like air movement around them...think about how they would be growing in their native environment. They grow at cooler, higher altitudes on the sides of mountains in areas around Capetown, South Africa...I would imagine there are nice breezes where they grow. Your temperature of 25C sounds good for them.

As for the ping....I'll steal a famous statement from Panman's avatar and say "I know nothink!". Can't help you with that one, but thinking about Mexico it does seem to be a drier climate than the mountains of South Africa. FWIW

Terrariums are cool, but not for most carnivorous plants...definitely not a good project for a newbie (like me) to get started growing CPs. I have thought about making one up as a moss garden, though. If I do it I'll probably use a large jar of some type. I've got a specific spot where my sister and I use to play. There is a short, carpet moss that grows there. I'm thinking of harvesting a bit of moss from there (family property) and making her a surprise up for Christmas. Basically a moss garden with maybe a succulent of some kind for added interest and a rock or two.

And lastly... "Welcome to the forum!" and "Check out the seed bank!".

Mexican pings need a much more open soil than your typical sundew. Soils mixed especially for them tend to be made up of ingredients like crushed coral, perlite, large sand and/or turface. No finer stuff like peat to clog the air transfer. A lot of people have even taken to growing them on top of chunks of porous rock. So I wouldn't expect your mexi-ping to do too well in the current medium. If it's just been re-potted, you might be able to change the medium out without further shocking it. Otherwise, I'd probably leave it as is and be very careful about watering it until it's recovered enough for a re-pot into better media.

Those aren't roots, they're plantlets. You're well on your way to replacing the dead ping.

I'd leave them alone for a while. Once they get a bit bigger and you start to see root buds forming, I'd move the whole leaf to a finer grained version of ping media, being sure that the rootlets are in gentle contact with the soil. Or, you could try moving the whole leaf now, if you're impatient.

The simple answer is yes. But don't go overboard trying to bury every root. Put the leaf on the soil and gently use a toothpick to press the roots into the soil. You don't have to get them all and you can try sprinkling some soil on the ones that are above ground. You're just trying to be sure the plantlets are getting some moisture. As they grow, they'll send out more directional roots and the above ground ones will become less unimportant.