to be honest, if your going to do a 5 gallon, might as well make it reef. you could get some really neat corals and such, but fish wise, you can really only have one, for example, a baby fire fish, green chromis, nano gobbys, or blue-tail damsels. but if you do decide to get a fish get them as babies so they wont get to big for the tank. Also get a rather large filter for the size, maybe a 20 gallon filtration system to keep the water as clean as possible, and do plenty of water changes! I hope this helped & if you have anymore questions that I didn't cover just let me know, and I'll try my best to help!
For a Reef tank a 5 gallon tank would actually be excellent if you are dedicated enoufgh to maintain proper water quality as in a tank this small things can go from good to bad in a very short amount of time.
I would suggest a 10 lbs bag of Live Sand, 5-8 lbs of choice Live Rock, AquaClear 20 filter, 25 watt heater and a Nova Extreme T-5 dual tube light fixture. This combo of componets will easily accommodate any soft or LPS coral that is photosythetic (requires light).
In fact I currently have such a tank going now with the reminents of whats left from my three other tanks after the flood and evacuations I had in October/November.
For fish, this is really not a good idea unless its one of the smaller Gobies/Pistol Shrimp combos which would actually do very nice in such a small tank. As far as Damsels I think that is not such a good idea as most can get to be 3.5-6 inches in length and quiet obnoxious to boot as well.
For a Clean Up Crew, a Pepermint Shrimp, 6-8 Astreas, 4 Cerith/Nassarious Snails and about 10 Dwarf Blue Leg Hermits or 3-4 Scarlet Reef Hermits should be the hot ticket.
You can pretty much disregard the need for a skimmer on a tank this small as 1 gallon weekly water changes should be more than suffcient. Besides there are very few quality units for a tank this small and they take up a considerable amount of space inside the tank.
Maintaining water quality will by far be your biggest challenge. No fish in the tank will make doing so much easier for you especially if new to the hobby.
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