I think it is a Hygrophila There are quite a few types, the giant one, (H. corymbosa) grows to 50 cm or more, Hygrophila corymbosa ‘siamensis’ which has smaller leaves grows to about 30cm. Both are very hardy, quite fast growing need moderate to high light, they generally prefer fairly hard water. Anything between 20 or 28C is fine for them. It should grow. If it gets too big you can just take cuttings off the top and plant them. There is one called Hygrophiala guianensis which is generally harder to grow. You could look these and the one Planter suggested up on tropica website to see which is the more likely species.