There are three major ones, plus more!

1. ST depression
2. Flattened T waves
3. Abnormally prominent U waves

The way to remember this is that, in line with the subpar nature of hypokalaemia, everything is weak and low!

ST depression is a depression, so it is low.

The T waves have low amplitude, so they too are low.

The dip of the letter U, as in U waves, looks like the minimum point of a parabola, so it’s also low.

Conveniently, it all follows an alphabetical pattern: ST, T, U.

References

1. ECG Learning Center. (n.d.). 12. Nice Seeing “U” Again. [online] Available at: https://ecg.utah.edu/lesson/12 [Accessed 4 Jul. 2018].
2. Burns, E. (n.d.). Hypokalaemia. [online] LITFL. Available at: https://lifeinthefastlane.com/ecg-library/basics/hypokalaemia/ [Accessed 4 Jul. 2018].