Start with a small CUC and add as the bioload increases. For now you should add a source of ammonia to the tank and look for spikes to see how well the tank can handle an increase in bioload, or if it is actually cycled or not.
I like snails for a CUC.
Astrea Snail-most common of all saltwater tank snails. They are excellent algae eaters and will forage all over the rock, sand, and glass. They fall very easily and can not right themselves up and then die unless you right them.
Banded Trochus Snail - Large snails that are excellent algae eaters although are very slow about it. They guys have a hard time picking themselves up if stranded in the middle of the sand.
Bumblebee snails- Not very hardy. Marginal algae eaters, but more of a detritus grazer. They can prey on other snails and sand bed critters.
Cerith Snails (Cortez) - Good algae and detritus eaters that forage rock, glass, and sand. Some can pick themselves up and some cant.
Cerith Snails (White)- Good algae and detritus eaters that seem to stay in the sand more than the cortez, but can be found on the rock and glass. Once again, some can pick themselves up and others cant. They are good sand bed snails to shift the substrate around.
Conch Snails - These are all about the sand bed and are very cool to watch. They usually dont get flipped over, but if they do, they can kick around and turn themselves back upright.They very hardy too.
Mexican Turbo Snails - super fast grazer that will knock out some algae in no time. They mostly stay on rocks and glass, but will work the top of the sandbed too. These may be cold water snails that dont do so well in a reef. But, once they are adjusted... they seem to be very hardy and active. Mine are going on two years and are huge now. I originally got them for a hair algae problem. They do tend to knock over anything that is not glued in place (corals, clams, rocks, etc.)
Nassarius Snails - They mostly stay in the sand, but will sometimes make short trips up the glass. They are mainly detritus eaters and do an excellent job cleaning and shifting the sand. I think these are also some of the most hardy snails out there. These guys can flip themselves upright very easily and quickly. They are fun to watch as they come bursting out of the sand bed at feeding time.
Nerite Snails - Nerites feed on filamentous and film-forming cyanobacteria, and filamentous green algae, but the main diet of most nerites is diatoms. These guys like to crawl out of the water some. I recently found one that "jumped ship" and was on the outside rim of the tank. These are great snails, but also have trouble turning themselves back over. Mine usually end up in the sump where they can get above the water line and come back down when they want to.
Stomatella Snails - They are supposed to reproduce very well. No personal experience w/ them.
Strombus Snails (Strombus maculatus )these are excellent snails that reproduce very well. They seem to be mostly sandbed snails and will occasionally make there way up to about the 1-3 inch line of rock work.
Though, as mr X stated you will have brown diatoms and other algae in the first few months of the tank.
