The natural host anemone for Maroon clowns is the bubble-tip (Entacmaea Quadricolor). Fortunately they are one of the easier anemones to keep and captive bred specimens are readily available. I would recommend buying an aquacultured BTA not only b/c of environmental issues, but they are hardier and better adjusted to life in an aquarium. 5 wpg
is good, just make sure to keep your water in good shape and parameters stable. Good luck
Edit-I forgot to add that you should have a "mature" aquarium before adding any anemone. IMO
if the tank is atleast 6-8 months old a bubble-tip should be alright, but a year is ideal.