Maybe...
All anthias still like swimming space.

I know you can do one at least. I bet a trio is fine if you keep up with feeding, since you have an MP20.

what do powerheads have to do with swimming space?

Nothing. But anthias like good flow. I know you can do one sunburst in a biocube 29, so I think they would be fine in a 40 with good flow.

I'd say your best bet would be Pseudanthias parvirostris. It stays small and is relatively easy to keep. The downside is that it can become pretty aggressive once it turns male. Did you want to do a single individual or a group?

Here's a female:


And a male:


The easiest way to ID these guys is to look at the "eyebrows", or pink stripes above the eyes. They should meet at the top of the head to form an inverted V. In both males and females. LFSs call them "Sunset" or "Diadem" Anthias.

Sunbursts would be a good addition too but they may be a little shyer and harder to acclimate than the P. parvirostris.

I've also kept a P. ventralis group in a 30 cube (they only get 2" max), but I wouldn't really suggest anybody buy these for any size tank as they often drop dead within a few weeks for no apparent reason at all.

+1. I know that if you get the sunbursts too and have only them in the tank for a while, they will be less shy once you add peaceful fish.

IMO, it would be difficult to choose, they're all beautiful. If I had a large enough tank, they'd definately be on my stock list. But fellow N-R member, seahorsedreams, posted some pics of her trio of Red Saddled Anthias (Pseudanthias flavoguttatus). Here is a link and if you'll scroll down to post #13, you'll see her pics of them.

http://www.nano-reef.com/forums/index.php?...208204&st=0

It's dangerous to sit in an echo chamber on nano-reef and slowly edge towards trying to slightly justify more and more each time about how it just might be OK to keep one anthias, a small one, then a group of them if you feed a lot, and then definitely if you have a high flow pump, and then if you change your water a little more than usual...

These are all traps that you need to be careful of. Don't take all that's said here as truth, be careful, don't subject animals to a meaningless death if you can help it.

I'm not saying they can't be kept in such a small tank, but after seeing them in large tanks, videos of them in the wild, I'm afraid to keep any fish in any tank at all, let alone a nano.

Anyway, careful with anthias in nanos, especially more than one.

+1. It's doable but I don't think I would do it.

I just posted this link in the Coral Forum, but thought I would post this here as well. It's a vid of a 20 foot reef tank. It is anazing and shows schools of anthias. Check it out!

http://www.youtube.com/watch?v=n_N3PqJCCQw...layer_embedded#

thank you all, sorry went off topic Nemo. I talked to many locals and LFS employees, and they said Anthias do best in groups and its hard to keep them in groups in such a small tank. He also did inform me that with 3 feedings a day are a must since they use up a lot of energy, and need to be kept fed. As cool, as the Ventralis Anthias are (droooool), I am going to have to pass on them lol.