Hi mate it's just a bit trickier on a small tank.  THe process is the same as on a big tank however:

Put it on pulse mode,  set it to as fast as it will go and quite a high power. 

Wait about a 30s and watch the water line from a distance, hopefully you see the start of some movement in the water line.  If so then just keep doing tiny (1mm ish) adjustments on the dial and waiting 30 secs to see if you can make the wave stronger.

If no wave forms,  turn the dial down a larger touch (3mm) and repeat.

Putting a bit of masking tape or something at the water line can make it easier to see when a tiny wave starts to form.

It’s similar to finding the resonant frequency of something. When you find it, a tiny input of energy adds up to massive movement.