WHICH TANK FILLS FIRST? ft. Simone Giertz

I'm going to make my hypothesis before I watch the video:
If we're following real physics, water is 'incompressible', and with such small drain pipes, even with open topped tanks a ton of pressure is going to back up due to the water not being able to drain fast enough, and by my assumption, tank 1 should fill to the top before any others.