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How to introduce a pleco to a goldfish tank 
Ask how large the pleco will grow to be. 
Although most pet stores will sell plecos that are only a few inches in length, you should know that they can grow quite large. Fully grown plecos can reach up to two feet in length.
Choose your answer from:
[I] Check your tank's surface before you place the pleco into the tank. Make sure that your aquarium is opaque or has a drop-down tray so that your pleco is not exposed to water.;
[II] This can have a negative impact on their lifespan. Choose a tank with large walls and no air holes.;
[III] Make sure you cover more than twice the tank. If you find that your pleco grows too much, cover the entire tank with a blanket.;
[IV] The exact size of the pleco will vary depending on the species. Ask someone at the pet store how large the pleco will get before making a purchase.;
[IV]