Today i woke up to 2 dead corys, ammonia .25-.5 (believed to be due to the dead fish) no nitrite, 40ppm nitrate. Did a water change to remove ammonia. All other fish look quite healthy. I had noticed these corys acting strange for quite some time, such as lying on the bottom of the tank for a long time. Both of them also had some frayed fins, probably because of the other fish, But on one i noticed it before it died(still probably the other fish). I have guppies and tetras in the tank with them. my other 3 corys are swimming around as usual. what do you think may have caused this sudden death?