I got some lemon tetras (six was all the local shop had) based on Willie's recommendation of them working well for him with large water changes. Awhile after I added in the rummynose out of my community tank eight of them and one died of stress while netting them, which was odd. One lemon died a few days later so I have 5 lemons and 7 rummys and I would say 75% or more of the time, they're all schooling together. My rummys definitely are pale colored during/after water changes, but they color back up pretty well after the water change or perhaps while it's filling. They snap back to normal behavior in short order, so I don't think they're taking it too hard. I have rescued one from the sink in the basement where water change water drains to and have caught a few getting into my Python, so if it's some sort of automated system, that might be something to consider. I have cardinals in my community tank and I don't really like them because they just don't do much. They certainly don't school much at all especially compared to rummys. I will say that overall the colors of these mostly silverish fish seem kind of washed out in my setup with is beige painted bottom with light blue back. Not a great picture at all, but this is the night I added the rummys. My discus have never appeared particularly aggressive to each other except some minor chasing from time to time, but never ganging up or anything. I haven't even noticed that once I put the dithers in, but they don't really harass the tetras either. Unless it's something happening outside of the times I'm home to see it.
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I keep my tank at 85, it's 75 gallons with 8 Stendker discus grown out from 2.5" to about 5" now, daily 90% water changes. Nothing has been in the tank with them except sponge filters until the last month or so, meaning that you shouldn't take my information as worth much because I've only had discus for 7 months . I'm interested if anyone has done the emperors with discus, because I think those are pretty neat looking fish.