It grew like a weed in mine, I was using ADA Amazonia soil and dosing EI (on the light side looking back). Started off as a CO2 tank @2bps but the pic below was after 3 months with no co2 which I was surprised about
Big trim on the C. Helferi, due to emerged growth dying back, missed one though...
Added half a dozen snails, loads of food for them on the wood plus the glass, which I didn’t clean this time. Let’s see how they get on.
Are the plants still potted to stop the goldfish digging them up? If they are then they will do much better planted with some root tabs under them. It can be hard for them to spread out of the rock wool.
It's a bit of a cliché but hornwort is a great plant for coldwater aquariums as it happily floats making it impossible for the goldfish to dig up. Elodea is another one that does the same as is water sprite. I like paraguay's hygrophila suggestion as well.
I’ve added a few floating bunches of Elodea and Hornwort for now and will try planting the potted plants with root tabs at the next water change. I originally kept them potted so I could remove them and clean around them more easily. Happy to give anything a try
I might add some Hygrophila at the back if the fish leave the other plants alone - so far so good!