Ramshorn snails, and they did a number on the Ambulia. The plant was looking really good a month ago, those snails are a pain in the blank. You need to look for snail eggs (thin flat jelly with spots in) and remove them along with netting out any adult snails. You will have to do it every week or two just to get their numbers down, and if you get a chance, do what the Daleks on Dr Who do, exterminate them.
The 2 baby tetras might be big enough to live with the adults now, especially if you can sex them.