Greetings, I have an established 29g tank. It has been established for several months and recently my Zebra Danios keep getting picked off 1 by 1 with all other fish seemingly fine. They have been mysteriously dying about 1-2 weeks from each other 1 at a time. They show no signs or symptoms prior and they all go to the same spot in the tank (at the bottom behind the big bridge decor near the intake)and just hover there the entire time without eating until I find them in the intake. There are no signs of fin rot, ich, or pretty much anything else, and like I said all the other fish are swimming around happily. The tank is not over stocked as there are 12 fish equal in size to the danios in this tank. Perhaps they are just dying of old age? The Danios were the first fish in my tank almost 10 months ago. I have 7 live plants in it and was wondering if maybe oxygen and CO2 might be a culprit?