smaller plecos - clown pleco, bristlenose pleco, queen arabesque pleco... all usually can be found for $30 or less, sometimes incredibly cheap if they're locally bred. Depending on the tankmates, one of the above species could be happy in that 25. Silver Dollars get far too large for that tank, but I'm sure you'll be able to find something else as a centerpiece. Perhaps dwarf cichlids like apistogramma, kribensis, or rams?