Our local aquatic centre are selling freshwater shrimps. They said that they're usually used as fishfood for the larger saltwater fish, but they ended up with these somehow and they've done well in coldwater, so are now selling them as pets.
They're small and whitish and look like interesting additions.
I removed the old weed when I was tidying the tank and put it into a bowl in the kitchen window with some pebbles. There are lots of minute snails in it now
I don't know whether they've come from the watercress that I popped in there, or the fishtank weed.