20lbs in month is easy..Im assuming when you say competition you mwan bodybuilding show, so I don't know if this will be what you're looking for. Everyone above proboly assumes you mean 20lbs of fat, which obviously isn't possible. I used to cut disgusting amounts of weight for wrestling, difference is I did it in a week. Tournys would be on the weekends, starting monday I'd cut water consumptions go a minimum, and id eat a few coldcuts of meat every few hours. So I'd keep my metabolism up. Then 2 days before the tournament till the morning of, I'd completely cut out consumption of everything, I put on a layer of sweats, a sauna suit, rub up with albolene and sit in a sauna for 30min in, 5min out, till I got where I needed to be. Id cut an upwards of 17lbs in a week like that. But this is not a safe practice, attt allll, I'd pass out all the time, dizzy, weak, sick, renal failure numerous times, and had to compete everytime.
But that was in a week, with no supplements. With all these ntbm products ive never knew about, and a months time, it should be easy. Like you're doing, eat nothing but fish, small amounts of chicken, and nothing but water, slowly ramp down the amount the closer you get to competition, then the few days before shed all your water weight.
Don't know if this will help you with your competition or not, but i hope it does, i tried. good luck