Hi all, Can anybody point me to a good resource for learning more about fish illness? I have a specific question as well, but I'd like to learn more in general. I just started my first aquarium about a week ago using tetra safestart. I bought 6 ember tetras at that time. The first few days I noticed that 3-4 would hang out, and the other 2 would hide on their own. I've been testing my water parameters every day, and things seem ok there (ammonia never reached 0.5, was usually more in the 0-0.25 range, has now been at 0 for 3 days; never any nitrites; nitrates slowly climbing, now around 20ppm). Between last night and today I'm down from 6 to 4 ember tetras. One disappeared and I can't find it -- looked through everything, the plants, the filter, etc. The other one died this evening. These are the two who were hiding ever since buying them. They seemed abnormally skinny compared to the rest, and one at one point had a stringy looking white poop? I don't know what fish poop should look like so I don't know if that's noteworthy. None of the others have had the white poop. These two had very vibrant, dark orange coloration, so I dismissed it thinking perhaps it was a male vs female difference or something, but now I think not. The 4 that are still alive look very healthy. Full bellies, nice and active, eating well. I checked my water parameters again afterwards, and the water still looks good. 0 ammonia, 0 nitrites, ~20 nitrates. Nitrates have actually gone down a bit, I think thanks to my plants, or topping off water that had evaporated. The only thing that has changed is adding CO2. I added it yesterday for my plants and the pH has lowered a bit while it's turned on, from 8.0 to about 7.4. I have a drop checker and I'm trying not to make changes too quickly, so I didn't let it get fully green, just a green-blue. Could the CO2 have done it? Any ideas about what my fish had? Suggestions for medicine to keep on hand? Do I need to worry about my 4 other fish? The saddest part is that I could tell this little fish's health was taking a turn for the worst so I ran to the store to grab some API General Cure wondering if it might help, but I got home right as my fish was dying and didn't get to try it. Literally watched him open his mouth for the last few times. So sad. Trying to figure out what I might have done wrong. I know I did overfeed the first couple days 'cause those two weren't eating much.