I would describe the more basic aspects of the saltwater hobby (keeping fish, softies, lps, and maaaaybe some of the easier sps), more of a diligence and handwork thing compared to a task difficult in complexity. I am on my first tank which is 30 gallons and sumpless, obviously a fair bit larger than a 10 gallon, but a smaller system none the less. I did manuel top offs for about a year before constructing an ATO this summer. Manuel top offs are slightly annoying, but if your willing to do them an ATO isn't needed. I would have continued with manuel top offs, but the thing spurring my decision to make an ATO was a long vacation. I wouldn't have been able to top off the tank myself since I was away, and I didn't feel comfortable with the person we had feeding the fish to put in the correct quantity of top off water in the tank. Making an ATO allowed the tank to top off while I was away and it also turned out to be nice because it allowed me to dose kalkwasser and keep some stoney corals. If you can handle adding top off water daily, maybe even twice daily depending on the season, region, and weather, you can do a fun 10 gallon tank. I definitely wouldn't put any corals too complex in that tank. softies, maybe an acan or some of the other less aggressive lps corals since space is precious in a tank that small. Maybe florida ricordeas and even if your feeling a little more ambitious and able to dedicate more time, maybe even an easy to keep sps like a montipora if you want. Appropriate fish selection would be 1 small fish like a goby. If you use a salt like reef crystals high in calcium and change like 5 gallons a week (probably not all at once, to avoid to much shock to the inhabitants, maybe change like 2.5 gallons at the middle and end of the week) , matching the temperature and salinity of your display tank, you might likely be able to keep calcium and alkalinity up while keeping 1 or two stoney corals in the tank with some softies.