hmm. Any problems with any other live stock?
Nope. Roselines and cardinals are all fine.
I've never had luck keeping then long term with co2, but I have a few 2-3 inch ones I've had for a few years in my low tech tanks that are as soft as you can get with a GH of 1-2 and a kh of 0
Hmmm. Interesting. I know for sure ferts isn't an issue and from your story, hardness shouldn't be an issue either. So how do people do it? Unless the roselines are already picking the amanos off when they're only 2.5"? Their mouths don't seem big enough to pick at amanos yet.
Also, is there a point of not cleaning your sump that O2 exchange isn't as efficient anymore?