In my opinion, 20 gallons is not enough space for a fish that can reach 9-10". And like most loaches, dojos are social fish that do best with company of their own kind, so buying "
a dojo loach" is not advised.
Loaches.com has two good articles on this species (both worth reading in their entirety), with the following quotes:
Quote:
Because of their sociability, they should not be kept singly as they like to sit in groups when at rest. They will often sidle up against one another and feel one another with their barbels.
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A group of three should be the minimum and even that small of a group will need quite a spacious aquarium. This should have a secure top as the fish are great escape artists.
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Quote:
Adults can reach a length of up to 25cm (10”), with slightly larger wild specimens reported. This size dictates the proportions of the aquarium that they should be housed in as being a minimum of 36” long when juvenile, and at least 48” long when adult. The tank should also be as wide as possible, so that the fish have a large ‘footprint’ in which to explore, and well-covered, to prevent these expert jumpers from escaping.
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I found this information on them (it lists tank size, too):