Hi Libby,
your big tank is 35 gallons. Your smaller tank is 15 galllons.
Neither is suitable for a common Pleco if that is what you have.
You could easily return them to the pet store for credit,
and get some smaller algae eaters.
There are tons!
Any that grow no larger than 6 inches would be suitable
for your 35g tank, and any that grow no bigger than 3-4 inches
would be suitable for your 15g tank.
For all you catfish and pleco lovers or people who are just plain
interested, here is a great place to look up the one you have,
the one you want, or to just dream of the day.
http://www.planetcatfish.com
Click on the cat-elog for listings of catfish. Many great articles on specific
kitties too in the catfish of the month journal.