You don't need to keep moss free floating in the water, attach it to something with superglue gel or thread.
But really, I wouldn't be trying any plants with goldfish, unless you have the money to keep replacing them, or have a way to grow them outside of the tank.
I'm also not sure how long that tank will be viable to hold those fish long term, I suspect that the tank will be overstocked based on bioload, and overcrowded based on the size of the fish and dimensions of the tank.
Or a sponge will stop stuff getting into the filter intake, if necessary, personally I'd only use it to protect fry or shrimplets though
But really, I wouldn't be trying any plants with goldfish, unless you have the money to keep replacing them, or have a way to grow them outside of the tank.
I'm also not sure how long that tank will be viable to hold those fish long term, I suspect that the tank will be overstocked based on bioload, and overcrowded based on the size of the fish and dimensions of the tank.
Or a sponge will stop stuff getting into the filter intake, if necessary, personally I'd only use it to protect fry or shrimplets though
