... About to invest in an anemone. I have a 20 gallon nano. ...
If you must have an anemone get some mushrooms or a leather coral or something similar. A large anemone such as a BTA would give you a lot of problems for a number of reasons.
First off you have a small tank and it's tall. Anemones like a BTA get fairly large and they will wander. In a tank like this it's just about certain that it will wander into a filter or power head intake, creating a real mess.
Large anemones also require excellent stable water conditions. While it's not impossible to do this in a 20 gal tank, it's fairly difficult. You just don't have the water volume.
Lighting is actually the least of your problems. It's low, but that's easily fixed with a lighting upgrade.
I know everyone seems to want an anemone to go with their clownfish, but this usually not a good idea. Pick one of the other options I mentioned. The clown may not host in them, but you'll avoid all the problems of trying to keep an anemone in such a small tank.