This thread has gone all over the place with citations from I don't know where, so it may help to bring it back on focus.
The minimum sized tank for a group of clown loaches is 6 feet (180 cm). An 8-foot is preferable, but it can work in a 6 foot. But nothing with less length. Water volume can be somewhat compensated for with more frequent and/or large volume water changes, but the length cannot. And any fish that is going (or should be expected to) reach from 200 mm (8 inches) to 305 mm (12 inches) cannot responsibly be in a tank less than 6 feet (180 cm) long before it reaches this size. As the reliable sites will mention, smaller tanks like a 4-foot and 5-foot are workable for young fish, but they must be moved into the larger space and as soon as possible. Fish grow continually, in two ways; externally (physical size) and internally (organ development). In small spaces without sufficient water volume they will not develop properly, at worst what is termed stunting, but even before this extreme stage there can be developmental failures that cannot be rectified later.
As for numbers, five is the preferred minimum, but four can work. It is not what we would recommend initially, by which I mean anyone who decides they want this species is expected to obtain five minimum, six or more if space will allow. And here we mean mature space; thinking you can get seven clown loaches now and put them in your 3 or 4 foot tank and then get a larger tank some day, is irresponsible. The larger tank may never materialize for any number of reasons. And as the fish's development is on-going all their lives, this practice is simply cruel to the fish. This applies to any fish; without adequate tank space for the fish at maturity, do not acquire the species. This simple concept can save a lot of fish.
I said four can work; if they have been together for some time, it is better to leave things rather than attempting to introduce new fish. Loaches are highly social fish. But they also establish a definite hierarchy within a group, and this will occur very early when a specific number are placed together in a new environment. This is unlikely to occur in the store tank, as the fish are very young, and way too many in the tank. But once you bring home four, five or six, within weeks they will establish their pecking order. Newcomers are likely to be driven off...except that is usually not possible in even the largest home aquarium, so worse results.
Byron.
