in responce to your question on size of tank needed. (found on guppy thread)

Although I would go with largest possible tank size - this article states no smaller then 50 gallons.... That is because they are extremely active and need room to swim (although some say smaller, the larger the better. 20 gallon is way to small for a fish that can grow up to 6-8 inches and is active. Also, since they are minnows think streamlined swimming ie fast, active. Semi agressive - please do some research (below link good start) before buying them and tankmates.

http://www.peteducation.com/article.cfm?cls=16&cat=1953&articleid=2602

Quick Stats: Rainbow Shark

Family: Cyprinidae

Range: Thailand

Size: Up to 6 inches

Diet: Omnivore

Tank Set-up: Freshwater: Plants, rocks, driftwood

Tank Conditions: 72-79°F; pH 6.5-7.5; dH 10-15

Minimum Tank Capacity: 50 gallons

Light: Medium

Temperament: Semi-aggressive

Swimming Level: No specific level

Care Level: Moderate

Reproduction: Egg Layer

The Rainbow Shark, otherwise known as the Ruby Shark, or Red-Finned Shark, is a beautifully colored fish which is a dark gray to black with red fins. There is also an albino variety.

Epalzeorhynchos frenatus is great for the semi-aggressive community aquarium, as long as they are the sole shark and other tank mates are of similar size. It requires a large aquarium with driftwood, rocks, and spots of dense vegetation. This shark may set up territories around the aquarium. The Rainbow Shark will become very aggressive towards its own species.

Unfortunately, the breeding habits of the Rainbow Shark have not been documented.

An omnivore, the Rainbow Shark is not a particularly finicky eater. Flake food, freeze-dried bloodworms, and tubifex, as well as vegetable-based foods should be fed.

Ideal tank mates include:

Barbs

Danios

Gouramis

Rainbowfish

South American Cichlids

Loaches

Plecos

Scavenger Catfish of similar size

......................................

I had one once in a bow 70 gallon with other minnow types ie bali 'sharks'. They of course are in the minnow family as well. They are jumpers (both actually), extremely active so a longer tank with a cover is best. They can get large as well. This link is excellent for information from experienced aquarists.

Best Sherry

http://www.aquahobby.com/gallery/e_frenatus.php