I've been looking for a replacement pleco for my 20 gallon too! I have a sailfin pleco in my 20gallon high that I bought a few years ago before I knew how big it gets. It's around 8 inches now and growing. I need to find it a new home.
Anyways the two plecos I've been looking at are the clown plecos (panaque maccus) and the flash plecos (panaque sp.). For the size, they seem to be suitable for a 20 gallon. Although I've read that the flash pleco requires wood to munch on. Not sure if that's absolutely needed for it to survive or just a plus to have.
Clown Pleco (L104):
Size - 3.9inches
Water Temp - 23C to 28C
pH - 6.8 to 7.6
Flash Pleco (L204):
Size - 5.1inches
Water Temp - 24C to 30C
pH - 6.6 to 8.6
Those are just basic stats they require that I gathered from planetcatfish.com. Here are the two links:
Clown pleco -
<a href="http/www.planetcatfish.com/catelog/speci...?species_id=734" target="_blank">http/www.planetcatfish.com/catelog/speci...?species_id=734</a>
Flash pleco -
<a href="http/www.planetcatfish.com/catelog/speci...?species_id=126" target="_blank">http/www.planetcatfish.com/catelog/speci...?species_id=126</a>
You will be able to find info on bristlenose and pitbull plecos on the site too. They have everything catfish on that site.
If anyone has more info (ie personal experience) on the care of these two please share! I'd like to know more about them too.