Hi there CM
My 1st Reef Tank was only 18g and I didnt really have an awful lot of problems, but I did upgrade quiet quickly so it ended up costing me more in the long run, IMO if you do want to go for a Nano as your 1st set up, you will need to do a lot of reading on them, I will pm you with some links to 3 site's that will help you out.
Some of your biggest problems with Nano's are Evaporation, Salinity, Temp & Nutrients Export, these can all be over come by topping up regularly, & checking reading daily, Fitting a PC fan will help the temp & regular water changes will help export any crap in the tank as you probably wont have a skimmer on a tank that size.
The other thing is that once you start & have 1 fish you will be hooked (so to speak) and will want to upgrade.
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