First of all, you do not need an Anemone for you to be successful with Clownfish. Moreover, Clowns will host everything from a Tridacna Clams Mantle, to Anthelia, Star Polyps etc.
Anemones are not the easiest of inverts to maintain. The easiest would be the Mojano and Aiptasia nuisance anemones... LOL. Sorry for the joke- these are nasty and you never want them in your tank! Haitian, Curly Cue Anemones are the easiest ones, which Clowns don’t host for the most part. Next would be the Ultra Rock Flower Anemones. Again, not hosted by Clownfish, but they add nice color and variety to a tank. Tube Anemones would be next, however, you would never see a Clown hosting these beauties as no fish could live in a symbiotic relationship with them. After that comes your Bubble Tip and Long Tentacle Anemones followed by the Sebae and Carpets being the most difficult.
Anemones need strong lighting, moderate water flow and optimal water conditions to survive. They need to be fed once or twice a week it’s silversides, clam, squid, krill or other suitable food stuff.
As for knowing when to remove an Anemone...when it shrinks and looks like a cottage cheese consistency or looks like it’s insides are being expelled through its mouth located in the center of its oral disk. Foul smell is also a tell tale sign that your anemones are dead and decaying.
You need to think hard about whether or not to add an anemones! Beautiful and fascinating...YES and the relationship between Clown and Anemone is downright amazing and a beautiful sight to see.
One last thing...just because you add an Anemone to your tank DOES NOT GUARANTEE THAT YOUR CLOWNS WILL HOST THE ANEMONE! It may take months, weeks or years before this carries out or may not at all. Also, certain Anemones are more Clown specific than others, meaning that a certain species of clown may be more suited to a symbiotic relationship with a certain Anemone than another.
I hope this helps!