I have been in the hobby for 30 years and by no means am I any sort of expert I am still learning and consider myself a novice. About 20 years ago I got into reef keeping by setting up a 75 gal aquarium. It was up for 10 years when I had to downsize and sell all my corals and an anomie which I had for 5 years. I kept the fish from that setup but all but one has moved on. The sole survivor is a cinnamon clown who is at lease 15 years old now I just added a yellow goby, 2 peppermint and one cleaner shrimp living in the 20 gal long that I want to setup as a nano reef. I have kept all the sand and base rock that I had in the 75. I understand that I will have to cycle the rock that I add to the tank and I will be adding an inch or so of the old sand bed to the under gravel sand bed which  is in place now, I will be removing the up tubes as to have a plenum of sorts. I will also set up the red sea Berlin protein skimmer. I want to add a calcium reactor with Co2 injection my question is do you see any problem with setting it inline prior to the skimmer or will that setup diminish the benefits of the calcium reactor? Now that I think of it would it be better to run the reactor inline from the skimmer effluent? That may give more contact time for the Co2 in calcium reactor.

I am not in any hurry to add the low light corals that I want. I am planning the setup as follows a plenum, live rock, protein skimmer, calcium reactor. The lighting will be 2: 20 watt N.O. (normal output) bulbs 1 aquatinic 1 daylight and 2: 9 watt power compact bulbs 1 aquatinic 1 daylight. I know this is under the 3 watt per gal for low light corals but it will have to do for a short time. I will be getting 2 more 20 watts the same type as are on now. The lights are 4 inches off the surface of the water. With the additional bulbs I could have the watts per gal at around 4.9

The only corals I want are polyps, mushrooms and the like with one exception, that being the elegance coral. What are your thoughts? Thank you in advance for your insight and experience.

Thanks Gene

With that said, I strongly suggest that you not put a calcium reactor on the 20-gallon tank. It's unnecessary -- you can't put enough hard corals or clams into a 20-gallon tank to require one -- and it's dangerous. It can and will cause great shifts in the alkalinity and pH in a tank of that size.

As for the Berlin skimmer, if it's the one I am thinking of, that's quite a bit oversized for that tank. The skimmer in itself is less of an issue than its pump. Do be sure you won't be creating a constant blast of water in the tank from the return of the skimmer, and do make sure any microbubbles are eliminated before the return water enters the tank.

The plenum is another thing you might want to reconsider. Realize you will be vertical wasting space in a tank with little height to begin with. Also, the claimed benefits of plenum use have been either in dispute or repudiated by a number of sources in recent years.

A Google search using the terms downside, of, plenum, in, tank turned up a number of discussions.

Glad to be of assistance. I happen to also have a 20-gallon (long) reef.