I have decided to dabble in ceviche, but it's a bit daunting. I have Tilapia fillets just for the purpose, along with the requisite citrus fruit (lemons, limes, oranges); cumin, red onions, shallots, balsamic, sea salt. I'm wondering if there is a rule of thumb for marinating time, thickness of the cut or strength of the citrus. I'd like to avoid undercooking without denaturing the protein. Does anyone have any ceviche tips or advice? Also, on a purely academic level, is there a certain PH to shoot for in the acid?