IMHO, if the tentacles are visibly vibrating, the flow isn't strong enough. I believe, this is an adaptation to being confined in stagnant tide pools. The vibrating tentacles create water movement, and assists in gas exchange while the anemone is confined in these stagnant pools. Once water movement increases, the vibrating stops.
If this was my anemone, I'd increase the PAR at least to the mid 300's. Gigantea is very prone to bleaching when lighting is insufficient IME. A very good tool to aid in acclimating an anemone like this, is to take pic's every day, with the same camera settings, and lighting. These anemones can fade very slowly, and may not be perceived by us until a drastic change has taken place. It's like watching your own children grow. It's not all that noticeable to the parents that see the child every day, but it's overwhelmingly obvious to the relative that only sees the child on holidays. When you have pic's, you can compare today's pic with a pic from last week, and any changes in color become much more obvious. This allows you to intervene and take corrective measures, before symptoms become to severe.