yeah, what nanook said....I must agree. IMHO, don't even bother trying to keep a crocea under fluor's. Get halides!! Plus, 420 watts sounds awfully slim for a tank of that size, again, all in my humble opinion. Croceas, aquacultured or wild collected, are the most light dependent of all the tridacna species. Maximas are close behind. You won't be sorry about purchasing a few halides (after your wallet recovers, anyway).