[Delphinus]

The fact of the matter is that you may well have done nothing wrong.

Anemones that are sold tend not to be captive clones, but rather, recently collected and imported specimens. These guys are suffering a lot of stress and a lot simply don't make the transition from the pristine wild to captive conditions. Those that do, tend to be very hardy, but many in fact most probably do not make it.

A new tank is undergoing a lot of flux, and it takes several months, really, for it to truly stabilize into anything resembling a sort of equilibrium. That's why some people (myself included) tend to recommend waiting 6 months to a year. There's also a secondary benefit, during those 6-12 months you're probably reading, plus experimenting on your tank, and so on, and at the end of that you're far better armed than you were at the beginning. I'm not saying that this is what happened here, I'm just explaining why I tell people to wait six months.

Trust me, your lighting did not kill the anemone, so eliminate that worry right away. Lighting (or lack thereof) can not kill within a couple of days.

The feeding the squid thing right away is maybe something I wouldn't have done. Squid is a heavy meaty food. I've found that with new specimens, due to the stress of collection/shipping/multiple-tanks-along-the-way-to-your-home, sometimes the metabolism just isn't there yet while in the adjustment period. The risk here is that the food may decompose faster than it can be digested, opening the door to some kind of opportunistic infection.

But then, it may have been doomed from the get-go. It's hard to say.

Anyhow I'm sorry for your loss, don't be too discouraged. Sometimes it just happens.