07-08-2011, 08:28 PM
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Yes, a 500 litre (130 gallon) tank is what you should have for the knifefish and sailfin pleco together. They would do fine together, with space to be themselves. If this is unrealistic for you (as it would be for me), I agree with bighugedome's suggestion to return the fish and select others that are better suited to your tank space.
This is a problem may aquarists find themselves in, especially as they first enter the hobby. All these beautiful and fascinating fish, 2-3 inches long, swimming around in a 20 gallon tank in the store--why not? But they grow, and grow fast some of them, and without sufficient space during that entire growing period, bad health is almost a certainty.
The sailfin pleco attains 15 inches, the BGK can attain 20 inches though usually slightly under that in aquaria. The former lives up to 20 years, the latter up to 10 years; so we are talking about rather long-term investments.