Egeria najas. Also known as Narrow-Leaf Anacharis, Brazilian Elodea and Egeria.
Egeria Najas is a fast growing bright green aquatic plant with small pointed leaves. It doesn't need much light but does need a temperature of at least 22°C (unlike Egeria Densa).
Anacharis is a great pondweed for school photosynthesis experiments as it produces a slow stream of easily counted bubbles, up to 40 a minute.
For more information on pond weed practicals in schools we recommend CLEAPSS' GL322 Guidance document.
Do not release into ponds or rivers as Anacharis is non-native.