I had great luck 2 years ago with my 28 gallon tank. I bought 6 medium and had about 70ish apts. I took about 2 weeks to eradicate them and was able to catch 5 for a friend when done. They turn white when they have not eaten. Sort of a greyish when they do eat. 2 years and still no aptasia! I also didn't introduce any new coral in that time.