Yes, even on the healthiest plant, a leaf will die occasionally. More sun is good, tho. I don't think there's anything you can actually do to this plant to hurt it except rot the roots. In which case you could still probably continue the top growth because these plants have an interesting ability to make a "stopping point" on their stem. I don't know what the technical name is for this thing but I've seen it happen, the top part just falls off, looking for a more suitable place to start new roots. This also happens sometimes if the stem gets pinched against the edge of the pot. The weight of a stem can cause a break/stopping point at the rim of the pot. Just stick it back in.

Speaking of disagreement, I count to ten whenever someone uses hardiness zones as a reference point for houseplant care. I too, am in zone 5, which merely tells me what the average minimum temperatures are outdoors in winter. That is what the zones were originally designated for, and as far as I am concerned, their only real use is for choosing appropriate outdoor plants. Zones say nothing about latitude and available light (there is a zone 7 in both Alaska and Texas, and a Zone 4 in both Maine and New Mexico, for extreme examples), or anything else having to do with indoor care. Even if you are looking for a last frost date, in preparation for putting houseplants outdoors, these are often the same across zones. One of the more memorable discussions I read online was started by a person who was astonished that her friend who lived in Zone 7 Virginia had the same last frost date as she, who lived in Zone 5 Kansas. That said, it just so happens that coincidentally, I have the perfect situation available for an experiment. I had a wandering jew that I found was doing rather poorly, due to a combination of not being in ideal light, and lax watering. Last week, I was just going to toss the whole thing out. But being a goofy, sentimental plant person, I couldn't manage to do so completely. ;-) So I took a few tip cuttings, which have already begun rooting. I cut off all the remaining leggy vines, and tossed them out. This left me with pot of soil with a few inch long stems. I was going to throw this out and clean the pot out for future use, but I haven't gotten around to it yet. If anyone is interested, I will hold onto to the pot, place it in a reasonably sunny spot, and see how it does. I will also takes photos of the process.

about rooting: you won't need to start this one in water. laying it on the soil and watering it every once in awhile is good enough. as buyorsell888 says, it's commonly grown as a groundcover in the SW and would appreciate as much sun a you can give it, else it'll become lanky and lose its strong purple color. it's also quite drought tolerant, so you may rot it if you keep it too wet. definitely keep an "insurance plant" around in the wintertime. they're very vigorous so if you start cuttings outside early in the year, you'll have a nice carpet at the end of the growing season. here in tucson, they take some cold weather (several freezes, a few weeks of night time temps in the 20s), but it isn't coupled with the wet like in the PNW. -ming

Once you've made sure the filter media is aquarium-safe, a lot of plant would work for this. If I understand what you said, the plants will have foliage above the water, roots in? Setcreasea pallida has been renamed Tradescantia pallida, just FYI. I'd guess the other plant you have is Tradescantia zebrina if there are purple/gray stripes on the leaves, Tradescantia fluminensis if they are plain green. The latter two plants will give you more longevity with this setup, IMO. The other plant is lanky, and prone to discarding more leaves along the longer stems, in a pot. Might not be such an issue in the water. Pothos and Philo are other fine candidates. Aroids in general do well with long-term water-roots, especially if there's some circulation in the water, like Anthurium, peace lily, Aglaonema, if you have the means to keep the crowns above water, roots partially submerged. I might also try sweet potato vines (Ipomoea,) various Plectranthus (that aren't prone to much blooming unless dropped flower bits would be OK with you,) Cordyline fruticosa (Ti plant.) Never had any Peperomia in water, so have no idea about that one. If you can post a pic of your set-up, other ideas might come to mind.