What type of pleco is it?? Is your 20 gallon a 20 long 30x12x12 or a 20 high 24x12x16??
If it is one of the Ancistrus species such as bristlenose most only grow up to 6" and would be fine in a 20 gallon long.
If, however it is a common pleco then yes it will outgrow even the 20 long.
