Oh no! Could there be some kind of tank/substrate issue from before this tank's redo? Copper or some untested for "other" thing? I remember you had home water issues at 1 time that you had addressed.
I have almost opposite water here 3 -4 gdh KH & GH, lower pH (7.4 ish) but mine also did poorly & eventually died off. Are there "bad strains" (overbred?) of RCS? But my last 1s after a mix of fire & super reds were mutts, (none bred or maybe even berried), so that shouldn't be the problem. Gee whiz, are others having these problems lately? RCS AIDS or something? (ok that was alarmist, sorry). I did use old Eco & plants from...everyone local.
I hope they do better in your 58g, fingers crossed!