A nano reef can run as small as 5-10 gallons but parameter stability gets harder the smaller you go. The sweet spot for beginners is 20-30 gallons. Sub-10-gallon "pico" reefs work but require daily attention, weekly water changes, and tight discipline on stocking. Below 5 gallons is impractical for live coral and fish together.
The smallest practical reef tank depends on what you mean by "practical." A pico reef under 5 gallons can technically hold a few zoanthids and a single cleaner shrimp, but maintaining stable parameters in that volume is a daily chore. A 20-gallon nano is where most reefers find a workable balance.
Possible but obsessive. Daily salinity top-off, weekly 25% water changes, hand-feeding precise amounts, no real fish (maybe a single sexy shrimp or a tiny goby). Best as a desk-side novelty for someone with a larger main tank already.
Workable for soft coral and a couple of zoanthids. AIO (all-in-one) tanks like the Innovative Marine Fusion 10 or the JBJ Nano Cube 12 are designed for this size. One or two small fish at most - a pair of Ocellaris is pushing it, a single small goby or fang blenny is more realistic.
The beginner sweet spot. Enough water volume to forgive small parameter mistakes, room for soft + LPS coral, can support 3-4 small fish, and the equipment ecosystem is mature - you can buy a complete kit and add coral within 6 weeks of setup. Most successful first-time reef builds are in this size range.
Salt creep on a 5-gallon tank can drop salinity 2 ppt in a week. The same water loss on a 60-gallon tank moves salinity 0.2 ppt - a tenth of the impact. Larger tanks dilute mistakes; smaller tanks compound them.