Do you mean the tank ran with the filter but no livestock at all ? If that is the case, that filter's bacteria would likely have died, and the filter may no longer be cycled. The bacteria need a constant supply of food, which is either fish waste or ammonia, to continue to live. They don't last very long at all without food.. a day or two maybe, but not three weeks.
If that's what happened, might want to double check and be sure the filter is actually cycled for the new fish. And unfortunately, tank water from established tanks does not contain biofilm or much of anything that shrimp eat. It's aged, is about all it is. It really won't do much, if anything, to help a filter become established. Used decor will have some biofilm on it, but not a whole lot, as would plants.
So the small tank with the shrimp, even though you put in used decor and tank water, won't have much of any biofilm until it has been running for months. Does it have a filter ? Sponge filters are small, and provide food resources when mature, which would take at least a few weeks running in an established tank.
Ghost shrimp are not really so terribly delicate. I'd even say they're pretty tough, but they are sold as feeders, rather than pets. Because of that they're rarely handled with much care while being shipped or held at stores, which is why some of them may die soon after you get them. But they also should be acclimated as for other shrimp, using drip acclimation is best and helps the most to prevent shock from different water conditions.
Most of those I get do survive and go on to have at least four or five clutches of eggs. They're in a 30G with other fish, snails and shrimp. Plenty of biofilm in there and I have a very large sponge prefilter on the filter intake, to stop anything being sucked up, which they can feed on. Nitrates around 20ppm, nitrites/ammonia zero. No heater, fans running most of the time to keep the temp down below 75.
I wish you luck with the shrimp but in a tank that's brand new, even if you had a cycled filter for it, they may not do very well. I hope you have good fortune. Keeping them as you might keep a Betta might possibly work, but I've never tried it. Don't think I would recommend it as a way to keep them for any length of time.
If you have some available, a piece of wood that's been in a tank for some time would provide biofilm, and you might also add some leaves, to provide natural food and perhaps help boost biofilm production.
You can use Indian Almond leaf, [ IAL], [ catappa tree leaf, an Asian tree], oak or beech leaves, even maple leaves, if they are gathered once they are brown & dry, from areas where no pesticides have been used. Sometimes you might find oak leaves still on the tree that are brown, those are great. Oaks hang onto their leaves much longer than most trees, some 'til the next spring.
You can boil them for an hour or so, or soak them for a day or two, if you have concerns about how clean they are. Then, place two or three leaves on the tank bottom and they'll soon produce bacterial colonies. They are not visible, and are entirely harmless but make very good eating for shrimp of most kinds.
When the leaves are merely skeletons, replace them.
I wish it wasn't so hard to keep these babies going!
