I think you should get a larger tank and adequate light before getting an anemone, or else your anemone just die. They are photosynthetic and require bright light to live.
None of the anemone that host 'Percula" clownfish (A. ocellaris or A. percula) are easy to keep even for expert. Of the 10 clownfish hosting anemone, BTA (E. quadricolor is the hardiest but it still require bright light and tank at least 50+ gallon to keep (it can get to to 10 + inche easy in good condition). BTA is not a natural host of "Percula" clownfish. There is no guarantee that your "Percula" will go into a BTA.
Adequate light and water paramenter, temperature stability are essential and these are very dificult to keep in a small tank with out bright light very good heater, chilling system and auto top off system. The high light requirement of these anemone cause wide swing in temperature in small tank as the light will add lots of heat in a small 2.5 g tank (thus cooling system is need when the light are on)
I keep anemone in a 24 g tank. I need 150 W MH light that will raise the temperature of my tank from 76 to 84 when it is on. I have to have heater on at night and fan cool to cool the tank during the day. Doing this I keep my tank stable at 80 degree. I have to add about 1 quart of water each day to comensate for evaporation. I will get a auto top-off water in the future so I can go out of town for a day or two with-out crashing the tank.
Clown fish do not need anemone. They can even breed in an anemone-less tank. Clownfish hosting anemone is not for beginner I would recommend against putting one in a 2.5 g.
If you still want to keep and anemone, please do research before you buy, and you will need to provide adequate enviroment for this animal before you buy.