Hi Sadielynn,
If you wish to use lemon juice, use a syringe and inject the aiptasia's steam with the lemon juice. The tricky part is to target the aiptasia's steam before it retracts.
I have read that it's effective, but really hard to inject them. The Joe's Juice is an easier method, because you don't need to get the needle inside of the anemone. I've had very good results w/ the Joe's Juice product, but never really tried the lemon juice method.
You can do the injection in the tank...neither the lemon juice nor the Joe's Juice will hurt anything but the aiptasia