I used to pay a particular company by purchase order for this exact reason. CC takes 2-3% of the payment, but purchase order - they’ve got to get themselves into the company system, track the PO, invoice, track the payment…at the time, a common estimate was $50 to process a PO, and if you’re only buying $100 batches, that’s a big hit. Did not like that company, but they were the only place to get whatever it was I had to buy.
Zebrafish need that rhythmic activity to form heart valves. They have unidirectional flow before the valves form, through some magic of resonance and pressure waves reflecting off arterial curves, and the resonance results in (IIRC) a stagnant spot where the valves will form. Do something to interfere with the mechanics and no valves form.