your water must be seriously laden with chlorine or nitrates/nitrites for your fish to have such problems in only 20 minutes. i would get a test kit and see what your reading are straight from the tap
ive forgot to add prime several times, and ive never had any ill effects. i even did it once with 2 stingrays in a tank, and they were fine. i feel for ya man, hope everything pulls out ok.
edit: if you keep posting all these updates, and this has all happened in no time at all, your water is pure chlorine and i would not be drinking it personally. either that, or soap or something got into the tank by accident. no water should do anything near this to your fish in 30 minutes.
if you claim your bichirs are losing slimecoat now, this is something much more than just not adding prime. theres tons of us that have forgot to add it at times im sure.