I agree with Iain, Ian and sparky. I kept cardinals in my 6 foot low tech tank. Used to use ro water at 28 degrees. they did ‘ok’.
When I broke this tank down I donated them to Dave at aquarium gardens, so were then in really hard water and much more heavily planted tank at the technically incorrect temp for cardinals of 23 degrees.
Within a couple of hours they looked fantastic. I’d never seen them look so good. They’ve looked brilliant over the last 8 months (they are old fish as well). It goes against all the books.
Just my opinion but a heavily planted tank with healthy plants does a lot more for fish’s health than lots of people credit.