Join Date: Oct 2017
Location: Canberra, Australia
You'll need to relocate the fish; I don't think you can replace substrate while keeping fish in the tank.
Also, 5 gallons is too small for a betta, pleco and corydora. Plecos grow huge, corydoras like to hang out in crowds. I'd say a 5 gallon tank is good for just the betta, otherwise none of those fish will last.