It is too early to transplant water spinach outside in Zone 5. This has been a cool Spring, and soil temperatures are too low. When the soil has warmed & night temps are consistently above 50F degrees (preferably above 60F) they can be transplanted. Until the plants go outside, you can root the longest shoots to multiply your plants. Every year, from the first harvest, I save the longest stems, root them, and put in a second row. I may only get a picking or two from that row, but it increases the yield per seed, and lets me freeze a batch or two before frost. As mentioned above, I sometimes root some shoots in a pot from the last harvest, and keep them over winter. If you do so, be sure to wash the stems thoroughly, and do not put that pot back outside. Last year, a few aphids survived, and despite all of my efforts to eradicate them, they multiplied. I eventually had to put the whole pot outside to freeze, lest the aphids spread to my other plants. The seed I mentioned above (years ago) is still going strong - it is at least 10 years old now. Got enough for the rest of my life, if I can keep it alive. But when it is dead, there will be no more water spinach for me, unless I can find another seed source. :-(

One out of 3 of my green malabar spinach seemed to survived the Florida winter. These are in small containers on my patio, which faces north and east, and quite shady during winter. It was cold here with a couple of light frosts. Anyway, 2 appear dead but the other had a thicker stem which appears to have survived and is now putting out its first leaf of spring. I was able to save about 15 fruits, but they are already promised to a few folks.

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.

Just saw your post. I'm thinking the seeds may be defective. I have grown MB spinach for the last three years and never had any issues. I'm in zone 7, mid TN. I always soak over night and plant directly in the garden. They do seem to take their time, I planted some about a week ago and just a few days ago started to see some results.