Hi there, Strangedude, and welcome to AA!
I bet you love your pleco, most people do. I love ours, but esp. a common, as I found the hard way, needs a MUCH bigger tank than a 20. They can grow to well over a foot long, I'm told. They probably need minimum 75, but bigger if possible. Also, let me give you a link to some articles about cycling:http://www.aquariumadvice.com/forums...es-154837.html
. These will be very helpful in understanding the nitrogen cycle, in your tanks.
And one more thing: almost NEVER, never change a filter. The filter is where the beneficial bacteria colonies form, and you need them. Always keep spare filter material cycling in the filter, and don't throw them away unless they are falling apart. You will find all this, and much more in those articles. Enjoy!! I know you will....I did.