What ATO do you use?

Hi all, looking into an auto top off. I know Tunze makes a great one for large tanks, but for $200, trying to find something a little more budget friendly if possible. What do other people use? Its going to fill my 50 gallon sump, Idk what size my resiervoir will be, maybe 20gallon, we'll see what fits. Can I use a nano ATO or no?
Thanks!